So I pulled out my OM-1 and noticed some strange looking goo in the bottom of the viewfinder. I determined it wasn't on the focus screen, but rather on the prism. A bit of searching found that this is a common problem. Olympus installed some foam in most of the OM1s as a light shield. It's apparently not really needed, and for a while the factory left it out. Then they went back to using it for the OM-1N. The problem is the foam deteriorates and turns into a tarry prism eating mess. And unfortunately once the foam eats the paint and silver off the prism there's not an easy way to repair the prism. I luckily had a spare OM-1 that had some other issues, but a clean prism. I followed the instructions at http://olympus.dementix.org/Hardware/tutorials/FoamRemoval/index.html and was able to swap the prisms. If you have an OM-1 it would be in your best interest to open the camera up and see if there is foam installed over the prism. If there is foam you should remove it before it kills your prism. It took me about 40 minutes to follow the directions to open the cameras and swap the prisms. It should be even quicker if you just need to remove the foam.