I had the same thing happen to another camera that was probably made by Cosina. I just looked at an OM2000 body on eBay and I see that it has the same design-flaw as my camera, which is: The metal tray holding the mirror does not support the mirror at the bottom. And the mirror is glued to that tray with a glue that is rubbery, similar to contact cement. Here's the problem: If the camera is stored with the lens horizontal, the mirror will be facing 45 degrees down. Thus, the mirror will gradually slide downward over the years, because the glue is behaving like a liquid.
Consequently, the bottom edge of the mirror will begin hitting the rear element of some lenses when they are focused at infinity, but not when focused closer.
The solution is to move the mirror back up, and then use a better glue. But I don't recall whether I tried this with my camera. I might have put the camera away out of disgust.
I recommend getting the OM1 repaired, as those are solid machines.