There is only one file type that uses the .OMOD extension:
Oblivion Mod Archive
Game file used by Oblivion Mod Manager (OBMM) and Nexus Mod Manager (NMM), two programs used for applying game mods to The Elder Scrolls: Oblivion and Fallout 3; contains the data files that alter regular game content; saved using .7Z compression.
OMOD files are used by game community members for creating custom game content. For example, a mod may change the properties of weapons or the appearance of scenery and characters.
NOTE: Both OBMM and NMM require that Oblivion be installed in order to apply mods.
Updated: January 11, 2012