If you had shutter capping you'd see a cutoff that's aligned to the frame and straight-edged -- and with this orientation, only with a horizontal travel focal plane shutter. As far as I can see, your OM-4 has a vertical travel shutter (like most metal-curtain types).

What you have here looks more like vignetting from a too-small or misaligned lens shade/hood. If you were using a zoom, it may show only on images taken at the wider end of the zoom range.

One (rather uncommon) way a shutter problem could cause this (especially at very high speeds) is if the two curtain edges are not parallel. This is hard to test (because your eye can't come close to seeing the shutter travel), but many smart phones can shoot "slow motion" -- that is, record video at a higher frame rate and play it back slower, sometimes even frame by frame.

If you have one of these (most better phones newer than 2018 or so should do it) you could (with camera back open) record the shutter travel at maximum frame rate, then examine the recording while running as slowly as you can -- this might let you verify if the leading and trailing curtain edges remain parallel throughout the exposure.

If they do, look elsewhere (hand alongside the lens, maybe, given you say no hood was used) -- if you see them misaligned, however, you may need to have the camera serviced or repaired.