This topic has come up recently.  IF your lenses have been adjusted properly for infinity focus, then when on an OM body, something on the horizon (or beyond a certain footage) should be in focus in the finder AND the film when the focus ring is on the infinity mark. 

Typical OM focal lengths and where infinity should start for them (applies to all lenses).  Specs I've read say 600X the focal length is infinity.  Doesn't matter  how far away you think something is.  A stop sign at the end of your block, a mailbox at the end of your driveway.  If it comes in focus at infinity, that is infinity for that lens (if lens and body are properly set).

 

                                  35mm lens    69 feet    or 20.8  meters or more

                                  50mm lens    98.4 feet or 29.7 meters or more

                                  85mm lens    167 feet or 42 meters or more

                                 100mm lens  196.8 feet or 59.4 meters or more

Things that can cause focus errors. 

 

1:  I've seen OM bodies with the tab broken off the focus screen.  This allows screen to be put in upside down, causing huge focus errors.  

2: Using the focus scale to focus instead of the finder.  Come on boys and girls, you kidding?

3: Bad eyesight.  Having untreated near sighted - far sightedness will cause you to not be able to see the focus screen clearly.  Focus errors abound!  You should be able to see the fine texture of the focus screen in the finder.  Remove lens, look through finder, can you see the sharp lines of the split image, the ring around the microprism collar (screen 1-13) and texture of the surrounding matte?  No?  Then you need contacts, glasses or lasik. 

4:  A camera that actually has a out of adjustment mirror.  Can be caused by impact damage or tampering. 

5: Camera missing it's pressure plate.  Film must be in firm contact with film plane for there to be any hopes of sharpness.  

Seriously, the BEST test is simply trying to focus on something on the horizon.  Believe it or not,  an object at 100 feet (or 29 meters) OR MORE, IS INFINITY for a 50mm lens mounted on a 35mm camera body.  If lens and body are properly adjusted and focus goes beyond this object, said object is NOT at infinity distance.  That's why a tall tree hundreds of feet away is the easiest test.

The mirror in the camera must be at a 45 degree angle to the focus screen. This insures that focus distance is the same to the focus screen AND the film plane.  If mirror is not at the proper angle, focus will be completely wrong at the film plane while it looks okay on the screen.  If angle is off, focus will be off for ALL lenses used on that body. 

Above specs subject to change without notice. 

The tip-off for a bad mirror angle is this:  mount your favorite lens, go outside and point camera at something you know is beyond the infinity distance for your lens as listed above.  Look through the finder and focus on that object.  Try a tree on the horizon or something similar.  If you can't focus far enough to bring infinity subject in, then miirror is wrong.  If you focus up to it and can go well beyond it and it goes out of focus again, then object is NOT at infinity distance or mirror angle is wrong.