Oblivion’s ini file is a text file that controls many in-game features and settings. It includes a long list of parameters and default values that the player can change using a text editor. Think of it as a very elaborate text-based settings menu. It can perhaps seem a little intimidating, but it's quite easy to make adjustments.

Why would you want to edit your ini file? To disable features that are just plain annoying (like intro movies), interfere with certain mods (like border regions), or are known to be buggy (like autosaves). To enable or improve features to take advantage of computing power that was uncommon when Oblivion was first released. And to generally tweak the game to one's personal taste.

Your Oblivion.ini file is found in your My Games folder, not your Oblivion folder. The Oblivion_default.ini file doesn't actually do anything and is there as a reference for ini tweakers. When you launch Oblivion or the Oblivion launcher, it generates a new ini file if it doesn't detect it. You can delete your ini file and use this method to create a fresh ini file if you need to.

You can easily find the settings you want to tweak using the search function of your text editor. Before editing a line in your ini file, I recommend putting a semicolon ; at the start of the line first. This "comments out" the line, making it unreadable by Oblivion but preserving the original value. Then copy the line without the semicolon and paste it below, then edit that line. That way you have a reference if you need to restore the original value. This way you can also quickly find tweaks you've made by searching for ;. If you use an advanced text editor like BBEdit, it will even color-code comment lines, making your changes even easier to find.

You can also consider a tool like BethINI for easily managing ini tweaks, though I found it wasn't an especially fine-grained tool.

Oblivion Reloaded tweaks.

The original version of this guide was part of my Oblivion Reloaded for Dummies guide, so we will start with OR's recommended ini tweaks. Not all of them are necessary! Read on to find out what they do.

uGridDistantTreeRange=30
uGridDistantCount=50

These settings are for how far your view distance goes. These are double the default values of 15 and 25. Alenet recommends these settings for Oblivion Reloaded's Distant View feature. Not only are they not necessary, they can cause instability, especially if you have RAEVWD and other distant-item mods installed. It may be best not to increase these values, and you could even experiment with reducing these to 12 and 20, or 9 and 15, for better performance.

iShadowMapResolution=512

Basically, the resolution of in-game shadows. Apparently, the default value of 256 is known to cause instability. 512 or even 1024 are apparently much more stable.

bUseWaterReflectionsStatics=1
bUseWaterReflectionsTrees=1

You may have guessed that these refer to whether water reflects trees and statics (i.e. rocks and similar inanimate objects). Leaving one or both of these set to 0 can improve performance. Try turning tree reflections on first.

fJumpAnimDelay=0.2500

This makes jumping less “floaty.” Works well with OR’s gravity setting.

fLODTreeMipMapLODBias=-0.5000
fLocalTreeMipMapLODBias=0.0000

The recommended changes make distant trees appear less 2-dimensional. Mind the “-” sign before the first value.

iPostProcessMillisecondsLoadingQueuedPriority=100
iPostProcessMilliseconds=25

The basic research I’ve attempted has yielded contradictory information as to what these do. I can’t tell you what they do or why these changes are recommended.

Basic ini Tweaks

These are settings I think any player should make.

SMainMenuMovieIntro=
SIntroSequence=

Remove the values of these settings to disable long starting videos. You can do the same for SOblivionIntro to disable the prologue video when you start a new game if you want, but it manages to reset unless you also change the name of OblivionIntro.bik in your /Data/Videos/ folder (I changed it to “disableOblivionIntro.bik").

bEnableBorderRegion=0

This allows you to explore beyond the borders of Cyrodiil, which is necessary for mods like Elsweyr DoA and some of the Unique Landscapes mods.

bAllowScriptedAutosave=0
bSaveOnTravel=0
bSaveOnWait=0
bSaveOnRest=0
bSaveOnInteriorExteriorSwitch=0

These settings disable autosaving in Oblivion. Autosaves are a source of save bloat in Oblivion, which can lead to instability. Consider a save mod if you’re worried that you’ll miss autosave.

bUse Joystick=0

Disabling this couldn’t possibly hurt if you’re using a keyboard and mouse, and it supposedly leads to stability and decreased mouse lag.

Other recommended tweaks

Here are some other lists of recommended ini tweak:

Alenet's Oblivion.ini guide
Oblivion.ini Tweaks from Prevent Crashes Oblivion guide
TESCOSI Oblivion.ini guide

I don’t necessarily endorse all of these, but they strike me as well-informed and reputable. The memory and threading tweaks found at www.tweakguides.com and other places are rather more dubious, unfortunately. The consensus seems to be that most of them at best they don’t actually do anything, so be aware.

Also, a word on iPreloadSizeLimit. Until recently, Oblivion Reloaded's setup instructions indicated that this setting should be quadrupled from the default. The Prevent Crashes guide linked above has a good explanation of why, based on older conventional wisdom. However, by examining reverse-engineered code, the OR team has recently concluded that this setting affects how intro videos are loaded, and nothing else. Thanks to olha2 and DianaTESGotH for mentioning this on Bevilex's forum.