I've used the Olympus eye cups on my OMs for a number of years, mainly to hold a corrective lens. The OM series has a fairly substantial eye relief, and I can see the whole viewfinder even when wearing glasses. Mounting a corrective lens in the eye cup holder (a fairly simple procedure with a plastic lens from an old pair of glasses, and a Dremel tool) makes it even easier to use.

The various eye cups will probably give different results. Eye cup 1 (the round one) looks like it would hold you farther away from the viewfinder window and be less flexible. Eye cup 2 (the rectangular one) appears to add almost no additional distance. My experience with the ellipsoidal one (eye cup 3?) is pretty much along the lines of what Zuikopath describes.