Here is another post about a project I found in the far ends of the internet ("here be dragons"), but which seems really promising never the less.
But first of all a disclamer by the original creator:
The screenshots you are about to see are not yet an eye candy, they're rather to be seen as a 'proof of concept' with lots of crappy placeholders. Work so far has mainly been done on the internal mechanics of game handling such as object interaction (player can carry gun which again can 'carry' a mag and the like), realistic calculation of trajectories, hit testing etc.So what am I talking about? A (most likely) open-source Operation Flashpoint type of game running on the Irrlicht engine with the current working title OpenFlashpoint:
All there is so far is a thread on the Irrlich forums (with a few more details and development screenshots) and sadly the main developer seems to be bogged down by "real-life" ATM. But it seems like a worthwhile project to support.
Oh and get this: it is developed primarily for Linux :D