The 1-8 Olympus focusing screen is the most popular for astrophotograpy. It has a completely matte screen with no Fresnel grading to distort the image or trick your eye into thinking it is well focused when it is not. When you reach focus with this screen you can be fairly confident that it will be in focus on the film as well. It is not nearly as bright as the 1-12, (standard screen in the OM1) and so framing your shots, and even seeing what you are photographing can sometimes be difficult, but it will be in good focus, which is the most important thing.
The 1-8 Screen new costs about $39. I will sell for $24. USD plus Shipping. Airmail $4.00 Surface $2.75