If haven't noticed, there is a substantial magnification difference between the OM1&2 and the OM3&4. The latter better for folks who wear glasses like the Nikons.
The OM4's metering is much wider then all other cameras todate except for the Pentax LX. This enables up to 2 minutes aperture priority autoexposure that no other camera - again except the Pentax LX, can do.
The OM4 also has spot metering (multi) not implemented in a manual focus Nikon slr.