Please, tell me how the hell you see either a G36, AUG or P90 in the OICW. Maybe take a look at the gun again.
http://www.popularmechanics.com/scie...e/1281176.html
Neither of those look anything like the OICW and the end product would look like shit. If you're going to do something, dont do it half-assed. It would need to be made from scratch. The only current existing airsoft parts that could be used is maybe the grip and magazines from an M4 and the selector switch from a G36.