OM1: walkaround, hiking, street etc. I go back and forth between the OM1 and an XA for these purposes. The XA is smaller and really very good but falls flat when it comes to any kind of shallow DOF portraiture so that is usually the deciding factor for which goes in my pocket. Actually I had an FM3a but couldn't justify it versus the oly after the prices spiked and they went out of production. I mean, the whole oly kit cost a tiny fraction of the price of the fm3a, body only.
F100: travel, sports, wildlife, always-in-the-bag backup. Not so good for macro because there is no MLU, and macro tends to put you right in the spot where it matters. The F100 is also very light, has no interchangeable screens nor waist-level option- these are all potential minuses for macro. But the big plus of the F100 is that you can slap on a vertical grip and voila, pretty formidable fast action camera. I used to have an F5 but the F100 almost totally displaced that so I sold it.
From time to time, I do something really nutty with my F100: I put a mamiya 645 format 80mm macro lens on there with an adapter. That works very well for portable macro i.e. roaming in the garden chasing bugs. If you aren't doing tabletop macro and have enough light then this mode works
very well. The mamiya lens is excellent and has a mag ring and floating front element, and the mamiya extension tubes are almost free. Actually, that whole combination is very sexy looking, especially if I slap the ring flash on there!
Tom, for macro I think you might well want an F3 or F4 with waistlevel and bellows. Look here:
http://www.mir.com.my/rb/photography/hardwares/nikonfinder/f4.htm
I am doing pretty much all macro with a bellows-focusing camera (rb or rz 67 or view camera). Helicoidal focusing and normal eyepoint prism VFs are total pains in the arse for macro, in my opinion. And I think TTL metering is a must for doing macro with these smaller cameras.