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.
I have an F100 as well and I adore it...but it isn't a small camera. It usually doesn't bother me but when I want to travel lighter I use my Nikon FE. It is significantly smaller than the F100 (it fits nicely into a small camera bag where the F100 won't). Nice viewfinder, Aperture priority, meter etc and you can still shoot with it if your battery dies, but the batteries seem to last forever. I also own an Olympus Om-10. It's slightly more compact than the FE, a nice camera but I prefer the metering of the Nikon, which is dead on almost always...the OM-10 metering isn't as reliable, more overexposure I find.
I won't ever sell my F100, nor my FE for that matter. Keep it if you can and add a smaller camera to your collection. I got my FE for less than $100 and the Olympus for something like $40, with the 50mm 1.4, a terrific lens. Just my opinion anyway.
I have a F100 and a OM1 with a 50/1.8. They are both wonderful cameras for different uses. If I want to have a camera in the bag "just in case", that's the OM1. For anything else that is even remotely planned (ie I know I'll take photos) the F100 is usually the first choice.