The following is a list of the most informative Oblivion Tweak Guides and most extensive Mod Databases to be found. This list may be updated with more links in the future, but I hope this helps for now. Enjoy!
TweakGuides.com
Oblivion Tweak Guide - A very informative source for just about all your tweaking needs.
AtomicPC.com.au
Optimising Oblivion v1.0 - Focuses on simple ini tweaks to improve both performance and visuals.
Optimising Oblivion v2.0 - A more indepth version of the guide focusing on performance and visual tweaks.
Optimising Oblivion v3.0 - A guide covering the use of mods, including an extensive list of some of the best around.
UESP.net
UESP Oblivion ini Settings - The Unofficial Elder Scrolls Pages Wiki section on ini settings.
UESP Mods Database - A highly organised and very extensive mods database Wiki.
TESSource.net/Oblivion-Files.com
TES-Source File Database - Probably the largest collection of Oblivion files and mods in one place.
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