Oblivion:Poison

thumb|right|A poison and two potions on a shelf in The Gilded Carafe
Poisons are harmful potions that can be used to damage enemies. In your inventory, poisons are listed in the "Potions" section using a green-colored vial icon (as opposed to pink for regular potions). Poisons can be applied to most weapons; when an enemy is struck with a poisoned weapon, the poison takes effect.

Using Poisons
Poisons can be applied to most weapons; the main exceptions are your fists and staves. To apply a poison to a weapon, simply activate the poison from your inventory. You will be asked whether you wish to apply the poison to your active weapon (i.e., the weapon that is currently selected). The poison will remain on that weapon until you attack an enemy, at which point the poison is transferred to the enemy and disappears from your weapon. You can immediately apply another poison to your weapon if you wish.

For melee weapons (blade weapons such as swords and blunt weapons such as maces and hammers), any poison remains on the weapon until the weapon actually makes contact with a target. If you miss the target (or strike a wall), the poison is not used up.

For marksman weapons (bows), the poison is applied to the bow, not the arrow. Once applied, it remains on the bow until an arrow is fired. The poison is removed from the bow upon firing, whether or not the arrow makes contact with a target. 

Notes on Using Poisoned Weapons
Applying poison to a weapon allows the weapon to bypass any immunity to normal weapons. Even if the enemy is immune to poison, the weapon itself will still be able to do damage.
Damage done by poisons is affected by Weakness to Poison and Resist Poison; Cure Poison can be used to cancel the effect of a poison.
Argonians and many undead enemies are naturally immune to poison. You must use Weakness to Poison in order to poison these enemies (or be affected by it, if you are playing as an Argonian).
The sneak attack bonus does not apply to any poisons on the weapon; it only affects the damage from the weapon itself.
Poisons can be used to do more than just damage your enemy's health. Magicka, Fatigue, and attributes also can be damaged. Powerful effects such as Silence and Paralyze are also available in poisons.
Poison effects, even from the same type of poison, 'stack', i.e. they are cumulative; they count as separate sources of damage, rather than a toggle between not poisoned/poisoned, as in some games. This means that you can apply a poison to every subsequent attack, allowing you to quickly load up a vast amount of damage on one enemy.
Poisons are not considered to originate with the player (or whoever used the poison). There are two main upsides to this:
Killing someone with poison does not count as murder. As far as any witnesses are concerned, the victim had a heart attack and died.
Poison is not affected by the difficulty slider. This is critical for any aspiring non-mage character on a higher difficulty.

Obtaining Poisons
Built-In Poisons
A variety of built-in poisons can be randomly found in various locations. Some enemies (for example, Bandits, Necromancers, and Goblins) will carry poisons; if you kill the enemies before they have a chance to use the poisons, you can acquire them. Poisons can also be randomly found in various loot chests. Alchemy merchants may randomly stock poisons, and there are a few other merchants (M'raaj-Dar, Shady Sam) with reliable supplies.

Custom Poisons
Poisons are made from ingredients using alchemy equipment, just like potions. The primary difference from regular potions is that poisons have negative (damaging) effects. Any combination of alchemy ingredients that have only negative effects will produce a poison. If the ingredients share even one positive effect, however, a potion will be created; the negative effects turn into side-effects of the potion. 
If an effect can be positive or negative (e.g. dispel), it is considered a positive effect for alchemical purposes.

Custom poisons can be far more powerful than the built-in poisons that you find in loot or buy from vendors. In particular, poisons combining multiple negative effects can be created. 

Custom poisons' damage is distributed over time, whereas built-in poisons do damage instantaneously. For example, custom potions with Damage Health+Fire+Shock and Damage Health+Fire+Frost effects (and Damage Health+Frost+Shock effects with SI ingredients) each do a maximum (with Master level equipment and skill) of 26 points of damage each second, but over 915 points of damage in total at the end of 38 seconds. Even if the stacking of additional doses of poison is desired, a hit and run strategy offers considerable advantages to the user of custom poisons, given the disparity between immediate and long-term damage.

Drain effects are generally not available for custom poisons: Drain Attribute effects and Drain Magicka are completely unavailable; Drain Health and Drain Fatigue are particularly difficult to obtain.

Unusual Poisons
Poisoned Apples and Chokeberries are technically food, not poisons. Like food, they are listed with all other ingredients and will be eaten when activated. NPCs will eat either Poisoned Apples or Chokeberries if they are present when an NPC is looking for food. Eating them triggers the poison, which in both cases is a script effect which continuously causes substantial damage until the character is dead. See the article on using poisoned apples for tips on how to kill NPCs.

The script effect that causes this damage can not be used to make a standard poison, i.e., you cannot create a green vial with the effect "Deadly Poison". You cannot apply the "Deadly Poison" effect to a weapon and strike someone. Both Poisoned Apples and Chokeberries can be used in alchemy, but the resulting special effect potions will be potions: pink vials that can only be drunk. If you activate the potion, your character will drink it and be killed by the Deadly Poison effect.

Notes
Certain NPCs carry poisons but do not always use them. For example, they are less likely to use poisons when ambushed, although they almost always poison their weapons if they see you first. This can be widely unpredictable in many cases.
In the rare case that an NPC poisons their weapon but dies before they can use it, you can loot the poisoned weapon. 
As already stated above, killing an NPC with a poison never counts as murder.

See Also
Poisons: Built-in poisons that you can find randomly in various locations
Alchemy: The skill used to make your own poisons
Poison Effects: The effects available when making your own poisons
Useful Poisons: Recipes and suggestions for making your own powerful poisons