Oblivion:Magic Overview

There are six schools of magic in Oblivion: Alteration, Conjuration, Destruction, Illusion, Mysticism, and Restoration.  Every spell belongs to one of these schools, and your ability to cast a given spell is determined by your skill level in that school.  The seventh magical skill is Alchemy, which allows you to create your own potions and poisons.  This article talks about general aspects of magic, including features common to all six spell-casting schools.



Types of Magic
There are several ways in which magic may be encountered in Tamriel.  All of these types of magic use the same Magical Effects, but differ in terms of how the magic is obtained and used.

Spells
Spells are widespread and easy to acquire.  Although the easiest spells can be cast by even beginning players, advanced spells require skill and Intelligence: casting a spell consumes Magicka, and spells can only be cast if your skill level in the spell's school is sufficiently high.

The spells that your character knows are all listed in your Journal under Spells in the All Magic tab.  To acquire more spells, you can purchase them from Spell Merchants.  You can also create your own custom spells if you have gained access to the Spell Making altars in the Arcane University or Frostcrag Spire.

All spells have skill requirements associated with them, meaning your skill in the magical school in question has to be at least that high to cast the spell.  These requirements are based upon your baseline skill level (without any Fortify Skill enchantments).  This applies to both vendor-sold spells and those created by the player.  Details on these skill requirements are provided at Spell Making.

The magicka cost of a spell is affected by your skill in the appropriate school.  Relative to the baseline costs (i.e., those shown on the Spells page) the actual cost of casting a spell can be calculated from:
 ActualCost = BaseCost * ( fMagicCasterSkillCostBase + ( fMagicCasterSkillCostMult * ( 1 - ( Skill / 100 ) ) ) )
Or using the default game settings:
 ActualCost = BaseCost * (1.4 - 0.012 * Skill)


Therefore, the baseline cost is equivalent to the cost at level 33.  At low levels, the cost will be higher (up to 40% higher at level 0); at high levels the cost will be lower (as low as one-fifth at level 100).  So don't despair early in the game if you find magicka costs for high-level spells overwhelmingly expensive. When you raise your skills enough to reach higher levels (e.g. Apprentice, Journeyman, etc.), all high-level spells will have become much cheaper in terms of magicka expenses.

The Skill in the cost equation is further modified by your Luck attribute.  For every 5 points of Luck above 50, your Skill is effectively increased by 2 points relative to the value shown on the Skills page, up to a maximum of 100 Skill.  Note that Intelligence, Willpower, and Personality have no impact on the cost of the spells from those schools of magic these attributes are related to (i.e., the total amount of magicka required to cast spells is determined by your skills, not by the attributes which govern those skills save your Luck).  Instead, Intelligence and Willpower affect your spellcasting abilities via your Magicka reserves:  Intelligence determines how much total magicka you have available while Willpower controls how quickly your magicka regenerates after you cast a spell.

The   figure on the Magic screen indicates the actual percentage of how effective your spells are.  The magnitude of a spell will be multiplied by this figure when you cast the spell; if the spell has no magnitude, then the duration will be affected instead (see Tips, below, for more detail).  So if your Spell Effectiveness is 85%, an Ease Burden spell will only allow you to carry 42 more points, instead of the full 50 points.  Your Spell Effectiveness will be reduced if you wear armor. There are two types of armor: light armor and heavy armor. It doesn't matter which one you actually use as far as your skill is concerned. Your overall Spell Effectiveness while in armor, either light or heavy, cannot exceed 95% regardless of your skill in that armor. The skill rating for the armor you wear must be at least Journeyman rank to maintain 95%; lower ratings than Journeyman results in lower effectiveness. 100% Spell Effectiveness can only be achieved if you are not wearing any armor (which is why most NPC spellcasters wear robes and no armor).  Fatigue levels do not affect overall Spell Effectiveness.

Casting speed is fixed for every kind of spell. However, casting while blocking is about two times faster than normal casting. It is invaluable for repetitive casting of ranged spells like fireball, or magic training. Do bear in mind that one does not need to hold down the block button in order to chain-cast. In fact, this may cause the notification windows on skill-up to not respond to your acknowledgements. Pressing the block and cast button/key at the same time is sufficient to chain-cast and avoid the aforementioned bug, although this is easier said than done depending on how one's controls are configured. It is also worth mentioning that if one has modified their controls to have blocking and casting be the same key/button pressing the one key will allow you to chain-cast. This can be done without losing the ability to block, as each can be assigned to different keys/buttons on the keyboard and/or mouse. Look at "Remap Mouse and Keyboard Buttons" on the Ini Settings page.

Casting any spell costs one point of Fatigue, although since fatigue constantly regenerates this effect is difficult to spot and almost inconsequential in typical gameplay.

Powers
Powers are similar to spells, but they cannot be purchased or created.  Several powers are bestowed upon characters based on race and birthsign; others can be obtained by visiting Doomstones, becoming a vampire, or completing certain quests.  Powers are listed in the All Magic tab of your Journal (under Powers instead of Spells), and are cast the same way as spells.  However, there are several important differences between Spells and Powers:
Skill level in the respective school of magic does not count against a power. A novice of Illusion, for example, can cast Lover's Kiss or Jone's Shadow despite the fact they contain Illusion spell effects that no novice could otherwise employ.  The magicka cost of a power is not affected by skill level.
Armor does not lower the Spell Effectiveness of Powers. This is one reason The Lord birthsign is advantageous to warrior types: it bestows a strong healing spell that retains its full effectiveness despite whatever armor they may be wearing.
Some Powers cannot be Silenced (notably the ones granted by birthsigns).
Some powers like the Vampiric Power Reign of Terror cannot be reflected or absorbed (but can still be resisted) by enemies, which makes them invaluable when fighting enemies that reflect/absorb spells.

Powers come in two varieties: Greater Powers and Lesser Powers.
Greater Powers can only be cast once a day, but cost no Magicka when you invoke them. Some Greater Powers are immune to  Silence, but not all. Notably, the Birthsign Powers are all immune to silence, but any Greater Powers granted by  Doomstones are not. Strangely enough, racial powers are mostly vulnerable to silence, the only two exceptions being the Khajiit's Eye of Fear ability and the Breton's Dragon Skin ability.  
Lesser Powers usually have a Magicka cost (with the Khajiit's Eye of Night being a notable exception), like spells, but they can be cast as often as those Magicka reserves permit. Lesser powers are also often (but not always) immune to silence. Eye of Night is immune to silence, allowing for 30 seconds of Night Eye at any time the player may need it. There are also enemies with silence immunities. For example, the Daedric elemental Atronachs are equipped with Lesser Powers, not spells, and are hence still able to use their magical attacks while silenced. Interestingly, silence immunity is not a unique option to just to Greater and Lesser Powers, but can be applied to generic spells through the Construction Set, and generic spells invulnerable to silence can be created via the Construction Set as well (although this should generally be avoided for balance). 

One disadvantage of powers is that casting them does not contribute any experience points towards your magic skills.

Abilities
Abilities are magical effects that are permanently (or nearly permanently) active on your character.  The most common abilities are those associated with races, birthsigns, and vampirism.  A limited number of additional abilities are associated with quests: some as skill bonuses, and some as attribute bonuses.  All of your abilities are listed in the Active Effects tab of your Journal.  All abilities are "on Self" effects.

A critical difference between Abilities and any other type of active effect is how Fortify and Drain effects are implemented.  Modifications from abilities alter the base value of the associated quantity (Health, Magicka, Fatigue, Attributes, or Skills) -- whereas all other Fortify and Drain effects only alter the current value of the quantity.  Implications of this difference include:
Fortify effects from abilities do not make the quantity appear in green in your Journal Stats; Drain abilities do not make it turn red.
For Fortify Skill abilities, you obtain any skill mastery perks from the skill increase.

If your character has lost all of his/her abilities, you have probably been affected by a bug that can occur during the Molag Bal quest or if you use Sheogorath's Protection.  Details are provided in the quest page Bugs section.

Diseases
Diseases are magical effects that remain active on your player until cured using Cure Disease.  Diseases are carried by various animals, undead, and NPCs.  You can become infected with a disease if you are struck by an infected creature.  Any diseases you have are listed in the Active Effects tab of your Journal.  More details are available on the Disease article.

Scrolls
Scrolls can be purchased from some merchants or can be randomly found in loot; it is not possible to make your own scrolls.  Anyone can use a scroll, regardless of magic skill, and casting a scroll does not use any Magicka.  However, each scroll can only be used once.  Also, using a scroll does not provide any magic experience.  All of the scrolls in your inventory are listed under Scrolls in the All Magic tab of your Journal.  They are used the same way as spells.

Comparison to Morrowind
Although the magic in Oblivion is very similar to that of Morrowind, some spells and effects have been changed a bit.  Key differences include:
Significant spells that are missing are: Mark/Recall, Levitate, Jump, Divine/Almsivi Intervention, Blind, Sanctuary and Sound.
The Enchant skill  is not present; instead, all enchanting is done either by using an Altar of Enchantment or a Sigil Stone.
Magicka regenerates constantly, making magic a more viable alternative to physical combat.
Casting spells can never fail, unlike in Morrowind where the failure rate was influenced by your skill rating in the relevant school of magic. Instead of a chance of spell failure, the cost of spells are influenced by skill ratings.  Also, Alchemy never fails in Oblivion.

Magic in Combat
Destruction magic is a powerful tool, but a dull knife wielded in certain hands is more deadly than a Daedric weapon wielded by an inexperienced player. Likewise, the enterprising mage must make certain decisions. Fighters and thieves are advised to use magic to augment their abilities rather than cause damage; those with such leanings do not need to face many of the following decisions.

First and foremost, get access to the Arcane University early: a Journeyman of Destruction would be foolish to cast a 6 point fireball. Purchasing spells from the Skingrad Mages Guild can alleviate this need, but self-made spells are generally more capable in nearly every respect.

Defense
Passive defense (as in armor) is an uphill struggle. Armor will decrease your spell effectiveness and Shield spells have a nasty tendency to give out at the worst possible moment. Shield enchantments, however, can replace armor to maintain spell effectiveness (and save you encumbrance). The highest your armor rating will reach is 85 and simple clothing enchanted with the Shield enchantment can provide the same protection as a full suit of Daedric heavy armor. It is highly recommended to join the Mages Guild and gain access to the Arcane University as quickly as possible. That way, you can enchant your clothing with the free soul gems from the guild halls.

Active defense, blocking and running, are the best ways to keep your blood in your veins rather than on the aforementioned weapons. With blocking, a shield may be preferable to a weapon for most classes, but the added weight and spell reduction makes a weapon the preferable tool; a simple dagger will block just fine and barely slow down your low-strength character. Unarmed blocking is not recommended, as that practice is essentially useless to those untrained in the appropriate skills. Likewise, don't use an enchanted weapon by default; you want to be able to repair your weapon to get that armorer skill up. If you need to use a weapon to attack rather than block, you want to be able to get back on the defensive FAST.
Simply put, a silver or Daedric dagger, upgrading to enchanted if/when your Armorer skill reaches Journeyman levels.

You can also use your Conjuration skill to cast bound Dagger or Sword to block with. Then once finished blocking carry on until you need to block next. This is a better way compared to carrying a dagger or a sword because it won't increase your encumbrance and will train your conjuration skill.

Aggressive Defense is another option, referring to the use of paralysis, demoralize, and calm effects to control your enemies. Illusionists may indeed find these tactics viable, but even with regeneration your Magicka is a finite resource when the situation turns ugly.

Conjuration provides decoys that the enemy creatures love to attack. Even a skeleton can provide a distraction at the critical moment. Healing is best done by Restoration at the end of any given skirmish, but hold onto potions of all types for emergencies.

Practically though, in the case of a mage, it appears that the best defense is offense - instead of wasting magicka on protecting oneself, use it up on frying the enemy before they can hurt you.

Offense
When casting spells, consider what resources you have, namely Magicka and spells. Your Magicka will last longer if you can give it time to regenerate, and the more Magicka you can throw around the more options you have. The more options you have, the better your chances of coming out of combat alive. For this reason, small amounts of damage over a long period of time are generally preferable to a single but largely damaging blast, and is also a fair bit more Magicka effective. On the other hand, one does not want to give the enemy time to regenerate. You must strike a balance. Five seconds between attacks seems to be a suitable compromise.

A good alternative is to throw Alchemy into your range of skills, then to produce large numbers of strong Restore Magicka potions. Create spells that drain your magicka dry in one massive blast distributed over 3-4s for better strength and time to regain your magicka. Weaker enemies will just go down after one hit, against the strongest ones gulp up 3-4 restore magicka potions and keep casting as often as you can, that is every 4-5s - except without worrying about running out of magicka and with delivering a several times stronger blast.

Choosing the spell to cast is also rather important. Many creatures are immune or have strong defenses to specific types of damage (fire, ice, shock, etc.). Some creatures are actually vulnerable to specific types of attacks. In a standard fight however, Absorb Health is likely the single best attack in the game; you will weaken your enemy while refilling your health, however this is only available as a Touch spell, not a Target spell.

If the enemy glows purple after being struck with your spell, then it has Resist/Reflect Magic in operation. In this case it might be better to switch to a physical (melee or poison) attack.

When engaged in melee combat, do not hold down the block key, as the enemy will likely perform a power attack. Rather watch carefully for their strike and block in time to meet the blow. The instant your enemy strikes you and recoils, cast your spell and prepare to block again. 

In ranged combat do not block. Wait until you see the enemy's bow or casting animation then dodge. Return fire with a powerful ranged spell. Alternatively, you can chain small spells indefinitely even at low levels. It is not as rapid fire as Morrowind's enchantments, but it gets the job done.

Complications arise when you face multiple enemies. In ranged combat, a nice duration attack spell makes the offensive complication trivial; just keep your distance and watch your bars. When engaged with multiple melee attackers, use summons to keep your enemies busy. This will likely distract just about all of your attackers. In case they do not take the bait, be prepared to retreat until your attackers stop following you. If you don't cast a summon spell in time, don't panic. Listen to your instincts, forget blocking, and focus on causing as much damage as possible. Keep a close eye on your health; Absorb Health is a good choice for this kind of situation and any kind of enchanted weapon in reserve can save your neck... or your tail, if you have one.

One useful hint is that it is possible to cast spells faster than normal. To do this, you must be blocking while you cast a spell--that is, you cannot simply hold L, there must a weapon or shield in front of you. The block will cut off the tail end of your casting animation, allowing you to cast spells almost twice as fast.

Cycling

A technique that has proved quite useful as a mage is to focus on creating an effective Magicka cycling system in combat. Cycling is performed by unloading heavy destruction/conjuration magic on your enemy quickly, but having a strategy to restore that magicka as fast as possible while remaining in combat.  There are some prerequisites needed to accomplish this:

Complete Azura's Daedric Quest to obtain Azura's Star Soul Gem.
Join the Mages Guild and gain access to the Arcane University so you may enchant items and create spells.
Create your own destruction spells for touch and ranged combat, and all of them must include Soul Trap for the duration of the damage done. If the damage is 1 second, that is fine. Soul Trap for two seconds will be all you need if you are dropping a one second bomb.
Create your own enchanted melee weapon with a grand soul that casts Soul Trap for one second, and also utilizes an Absorb Magicka effect.
Focus on gathering Spell Absorption equipment of all types.
Alternatively, either enchant a blade with Soul Trap, or kill Umbra at Vindasel and obtain her sword with the same name (which has Soul Trap 120 secs).

With the above, you will find yourself with one hell of a fury fighting mage. A traditional battle will occur like so:

Hit your enemy with a barrage of some heavy Destruction spells, preferably on touch (created by you as explained above).
Perhaps summon Daedra for backup
Have your weapon (explained above) at the ready
When your magicka gets low, start slashing away, and your magicka meter will quickly climb again
Rinse and Repeat until your enemy dies.

When they do, you will more than likely trap a soul, which Azura's Star will catch.
Use the soul immediately to recharge your weapon.

Rinse and Repeat.

With a good amount of spell absorption gear on top of this, the technique above becomes absolutely lethal against any enemies that use magic.

Other Concerns
Have a supply of enchanted rings and amulets at all times and switch them around when you need to.
Night Eye is priceless; Detect Life is even better.  Even well-lit caves have shadows a Clannfear can hide in.
Silence is deadly.  Silence effects come as both poisons and magic. 
Conversely, keep a stock of Cure Poison and Dispel potions on hand. The poison silences can be especially nasty, as some can last 30 seconds or more. These potions will get your vocal cords working again. 
The Fortify Strength enchantment is more cost-effective at the enchanting altar than Feather. A couple of rings of strength allow you to carry out all the loot after you've dealt with the dungeons baddies. If enchanting with anything higher than Descendent Sigil Stones, Feather is the better option on a point-for-point basis, but see discussion of the Feather bug before making any decisions.
Self-made potions tend to restore over a long period. Keep a supply of premade potions for emergencies, in particular Restore Health and Restore Magicka.
Use the Praxographical Center at the Arcane University to more efficiently train your magic skills.  See General Magic Strategies for general hints; specific recommendations on custom spells are provided on each skill page.
The combat AI frequently results in allies getting in the way of your shots; even if you have a clean line of fire, the enemy will try to dodge, again possibly resulting in hitting an ally.   When using target spells, you may wish to save often.  Or you can restrict on-target spells to early stages of the fights, then use touch spells once melee fighting has started.

Skill levels and training

Each of the six colleges of magic has five master perk levels: Novice, Apprentice, Journeyman, Expert, and Master.  The magic mastery perks determine which spells you are able to cast.  Every spell has a minimum mastery level required to cast it.  Your unfortified skill level must be at the required level before you can cast the spell.  In addition, the Magicka cost of every spell goes down as your skill level increases; see spell cost for details.

Increasing the spellcasting skills is easy. Just cast a spell over and over again. There is one requirement, though: the spell must hit a target. "On Self" spells will always hit a target. Ranged and touch spells need to strike a target that can be affected by the spell effect. Casting a fireball on a chest won't give you experience; casting an open spell on an NPC won't, either.

Tips

Weaknesses
Weakness effects (to magicka, elements, poison, etc.) apply to enemy effects from the next hit (with weapon or spell). These effects also stack and multiply damage done, and are important when you want to deal major damage.

Using Armor
Wearing armor reduces your spell effectiveness. The reduction penalty depends on your armor skill (light or heavy), and the penalty is reduced as your armor skill is increased. The penalty ranges from 20% (at about armor skill = 5) to 5% (at armor skill 50-100). The Magic Menu will show your spell effectiveness.
Wearing any armor reduces your casting effectiveness.
Exception: using a shield with your fists, a two-handed weapon, or bow gives you the shield enchant bonuses without the armor penalty or the shield armor class boost.
Because of the above point, it is still possible to have the Escutcheon of Chorrol shield equipped without having to put up with 95% efficiency. This allows a full-on mage to Reflect Damage 100% (with a ring and amulet, of course).
An alternative to using armor is to enchant clothing with a Shield or Elemental Shield effect.
Using a Grand Soul Gem, you can enchant a shirt, shoes, pants, and a hood for 10% each, giving you an Armor Rating of 40 (far more than you could get from a chain mail suit) or if your level is high enough, you can use Transcendent Sigil Stones for 25% each, giving you an Armor Rating of 85 (maximum possible). There is no magic penalty because none of those items are considered armor. See Useful Enchantments for more info and tips in this regard.
Armor points are capped at 85 (i.e. 85% Shield).
The armor penalty affects the magnitude of spells and is rounded down. If the spell has no magnitude (for example Paralysis), it will affect the duration of the spell.
So if you cast a 10-point damage spell for 10 seconds at 95% effectiveness, you will see only 9 points of damage per second for 10 seconds (90 total damage instead of 100).
If you cast Bound Bow, which has a duration 15 seconds and no magnitude, at 80% effectiveness, the conjured weapon will only exist for 12 seconds (15 x 80% = 12).
Since the recalculated values are always rounded down, a low-magnitude spell takes an especially large penalty. A 3-point healing spell for 120 seconds (an otherwise Magicka-efficient means to regenerate Health in non-combat situations) will only give you 2 points at 95% effectiveness, essentially taking away one-third of your full healing potential!
Reductions in effectiveness are especially troublesome for Illusion spells with a magnitude that refers to target level, and is capped at 25. Magnitude 25 spells are actually governed by an exception that allows them to affect all levels of enemy (see: Magnitude to Level Conversion). But because wearing armor reduces the magnitude to 95% of 25, which is 23.75, and rounds it down (to 23), the exception ceases to apply, and so targeted Illusion spells become nearly useless at high levels if wearing armor.
Spell efficiency reduction has the strongest negative effect while chaining spells.
Every spell in the chain is reduced by the efficiency ratio with every factor rounded down separately. 
For example, at 100 armor skill with 95% spell efficiency, when you cast Weakness to Fire 100% and followed by a Fire Damage 10 spell, you get only 17 fire damage. This is because first, the damage of the fire spell is 10 * 0.95 = 9.5 (which gets rounded down to 9). Then the weakness effect means the full damage is 9 + (9 * 0.95) = 17.55, which gets rounded down to 17.

Using Paralysis
Adding Paralysis effect for 1 sec to your spells gives you about 3 sec time to cast your next spell (or hit), because paralyzed creatures or NPCs mostly fall down and then take a while to get up. This is also a great effect when spell chaining.