Tes4Mod:Intro to Mods

Intro
Mods are modifications and expansions of the original game.  In the Oblivion mod community, the unmodified game is often referred to as the "vanilla" version. Mods revamp just about every part of the game. This introduction is the pre-step to using this site as a guide to overhaul Oblivion with many types of mods and keep the game running smoothly. When you download a mod, it will normally be contained in a compressed archive. Common examples of compressed archives are 7-zip (.7z), RAR (.rar) and ZIP (.zip) files. There are also two more archive formats that are specific to  OMOD (.omod) and Bethesda Softworks Archives (BSA) (.bsa). For now, the important thing to know is that these are used to package mods.

Important Locations
Mods are installed to the Oblivion Data folder (often just referred  to as the "Data" folder), a subfolder of the Oblivion install folder. For example, if Oblivion is installed to   then that is the install folder and   is the Data folder; or, if Oblivion is installed to   then the Data folder will be found in it.

This Data folder is the most frequently modified of the few folders Oblivion mod users need to use. However, many commonly-used utilities and extensions for Oblivion are installed to the Oblivion install folder itself, including the official construction set (TESCS) and Oblivion Script Extender (OBSE).  ''Unless you are installing or uninstalling one of those utilities, is best to leave the files in the install folder alone.

If you are unfamiliar with the location and the contents of the folders installed by Oblivion, please read Oblivion Installation - Installed Folders.  Also, if you are running Windows Vista or 7 and you have installed Oblivion (or Steam) to the default location (meaning you did not change the install location during setup) uninstall and then reinstall Oblivion to an unprotected folder (i.e., C:\Games) to avoid problems with running and installing mods and related utilities.

Mod Contents
Inside the archive you will find one or more of the following:''

Plugins

These contain the information the game engine needs to arrange all content in the game, from script activation to object placement.

ESM

Master files (.esm) are typically plugins that contain all new resources that are placed in the world and otherwise used via an ESP file. Of course, Oblivion.esm, the master file for the unmodified game is an exception. One advantage of placing many new records in a master file is that they can easily be used to provide resources for one or more ESP files, allowing for more flexibility and modularity in a whole mod's structure. For example, Common Oblivion (COBL) is one of the most well-known and widely-used library mods, the key being it master plugin, Cobl Main.esm (not to mention it was authored by the creator of Wrye Bash, having innate support in its Bashed Patch as well).

ESP

Regular plugin files (.esp) are almost always dependent plugins, dependent on a master file. Most are dependent upon the main master file for Oblivion, Oblivion.esm, but some mods include their own (i.e., COBL and Better Cities). If a mod does also contain a master file, the item placement and leveled list modification is usually only found in the ESP file(s). It is possible for regular plugins to be dependent upon other ESP files, which is often the case for patch mods and patch support in mods. (See TESCSWiki:De-Isolation Tutorial for more information.)

Configuration Files

Configuration files are files that are often read as a mod is loaded or initialized to set various parameters and values. Many such mods require that the configuration file be present and configured properly, others include it as an alternative or supplement to in-game--via a menu--configuration, and still others only include them as completely optional files for customizing values.

INI

These are text files, but the game reads them such that text on the same line following a semicolon (;) is ignored as a comment. This is the file format for most configuration files (as well as that of the game itself). A number of utilities, such as Wrye Bash also offer these kinds of plain text configuration files. They can be more convenient for configuration a mod or application, especially if the values need to be used every time but are rarely need to be changed (i.e., a file path to some tool on your computer or disabling invisible borders in the Oblivion.ini).

These can be edited with any text editor, but text editors that offer syntax highlighting (i.e., Notepad++) can make it easier to read these configuration files if they include comments. Comments often include default values and/or a description of the setting. If the comments are sparse or lacking, the ReadMe for the mod will usually include (further) instructions.

TXT

Rare, but thanks to the OBSE plugin Pluggy some mods have plain text (.txt) configuration files. Few mods use the format for as a main configuration file, but some use these to facilitate optional features that are customization-based. For example, if a user enables Enhanced Economy's custom prices feature, during initialization the mod will additionally read the TXT files (containing price data) from a custom directory,  , and then the user can choose one of these custom price data sets from an in-game menu (activated by pressing   in the player's inventory menu).

Mod Data Folders

Technically, a mod archive can contain just about any arrangement of folders, but the game itself only accesses files in the Data folder and the Oblivion folder in  . Mods seldom use the latter, but all mods, most often in their entirety, are installed to the Data folder.

Also note that many mods are not packed in the way they must ultimately be installed. Simpler mods are often packed such the archive includes a Data folder or any of the data file folders down inside an additional folder or two. More complex mods that offer multiple plugin configurations where plugin names are shared (i.e., two versions of the main plugin for those with and without OBSE) might instead include folders with helpful folder names containing the different copies of the plugins. Unfortunately, the actual package structure can vary a lot, but the recognizable data file folders should be found at some level. Hopefully, the modders which include loose texture files in some random folder with instructions about the location where the files need to be installed specified in the ReadMe are few.

:''Note: Pluggy allows modders to store and read plain text files in uncommon locations. If you are wondering where such mods are storing, for example, a PC's data, see the mods' ReadMes for the locations.

:Note: BAIN users, if a mod contains non-standard folder which need to be installed (i.e., configuration file folders), you need to right-click on the package in the Installers tab and flag it as having extra directories. The recognized folders are restricted to

Default Folders

After you install the game, most of these folders are packed away in BSA files, so you will not see them, but these are the folders the game recognizes without any extensions. You can save much trouble by backing up the Original Oblivion files after patching them. You actually may wish to back up both the patched and unpatched files. The files that users need to replace most often are probably those in the Shaders file, so some nice individuals have uploaded the official shaders online.

Meshes - NIF files
Music - MP3 files, WAV files for lip sync
Shaders - SDP files
Sound - MP3 files
Textures - DDS files
Video - BIK files

Additional Common Folders

These folders are a mix of installed resources loaded by the game itself or mods, OBSE-extended Oblivion, and files that are not loaded at all.

Docs - Mod documentation is available in many forms, including but not limited to DOC, HTML, PDF, RTF, TXT.
Most documentation files are accessible with any common editor or browser.
OBSE''' - OBSE extends Oblivion such that it is able to read the library files (DDL) in the   folder, which further extend or modify the game at a lower level. 
Nothing is installed to   itself, but, rather, a subfolder  . Feel free to use this parent folder to store plugin archives or anything else.
Due to the nature of DDL files, being able to modify the way the game executes certain operations, it is important to take heed of alpha and even beta statuses of these plugins' releases.
If an OBSE plugin remains in beta status, that does not mean that it is unstable, but you can read the comments, forum and/or download stats if you are feeling unsure about a release.
Alpha status for an OBSE plugin often indicates that testing is incomplete or insufficient by the author's standards. (This applies to regular mods as well.)
The status of an OBSE plugin may not be explicit or even applicable to the entire plugin in the case it includes a configuration file (as many do), so read the documentation for these mods carefully.
INI - In addition to the Data folder itself, these folders are folders a number of mod creators have agreed upon as being a convenient place to install INI configuration files so as to reduce clutter in the Data folder.

Other Folders

Many modders create resources or include resources with their mods that do not have to be installed at all. If you are having trouble figuring out which folders need to be installed, see the ReadMe; however, if the mod is BAIN-ready or OMOD-ready (as would be indicated in the mods download page and/or the ReadMe) installing the mod through the respective utility would account for whatever the actual package structure is.

See Also
Complete Installation Guide