.OPPC File

File extension oppc seems to be mainly associated with the Space Marine action video game by Relic Entertainment.

An .oppc file contains game resources, such as wargear, weapons, models and other elements used in the game.

The same format is also probably used in Darksiders computer game. Most likely shares same game engine.

How to open:

These .oppc files can be extracted with smoppc command line utility found on Space Marine modding forums (see links).

How to convert:

A conversion is unlikely, but these archives can be extracted.