Hi,
I've attached some picture of a complete OM1n Mirror box that may help, I'm not sure what state yours is in but I assue the mirro lifting spring has enough tension, also when you raise the mirro by finger soes it go all the way without any sticking or juttering? One other thing, when in the chassis the mirror lifting bumper may be degraded or misaligned, finally when it was in the camera did it lift to a certain point but not fire the shutter or just do not lift at all?
If it helps I could post the one in the picture, I'm in the UK, but if your outside the UK it may be cost prohibitive.