My Mamiya M645 prism had a similar problem. The foam's adhesive ate away at the mirror finish right in the middle of the viewing area. I removed the rest of the foam and attached a small mirror piece facing in to the prism. It works but you can see a slight band. The only other way is to have the prism re-mirrored. (I don't know if this is possible.)
It should be a lot easier to have the mirror replaced on your OM.
It is probably best to have the foam replaced before it turns to tar and effects the mirror surfaces.