I am a long time Olympus OM user (30+ years) with over 35 OM Zuiko lenses and a ton of other OM equipment and also own a Nikon FM3a and Nikon FE2 and several highly regarded Nikon lenses (17-35 f2.8, 28 f2.8 AIS, 105 f2.0 DC) as well as MD-12 motor drives and a Nikon SB800 flash. While I do not own an OM-2S or Nikon FE, I have handled and used them in the past. So I am pretty familiar with both systems.
Both systems are excellent in my opinion, being well made and durable. They both were plainly built better than the Canon A series cameras, with which I am also quite familiar. The advantages and disadvantages are as follows:
OM Advantages/Nikon Disadvantages:
1. Noticeably smaller and somewhat lighter.
2. OM-2s has TTL flash metering with a dedicated OM flash, the Nikon FE does not. (Nikon FM3a and FE2 have TTL flash metering)
3. OM-2s has spot metering in manual mode.
4. OM viewfinders offer better coverage (97% vs. 93%) and are brighter than FE viewfinder. If you get a rare OM Series 2 focusing screen, the OM viewfinders are much better than Nikon FE, and are meaningfully better than Nikon FM3a with newest K3 screen.
5. OM Motor Drive 1 and 2 offer five frames per second film advance with OM-1, OM-2 and OM-2👎, OM-3, OM-3Ti, OM-4, OM-4T, OM-4Ti, while Nikon MD-12 only offers 3.5 frames per second. However, OM-2s only goes to 3.5 frames per second.
6. I prefer OM viewfinder display, as it is obvious from the display as to whether the camera is in manual or automatic mode. With Nikon viewfinder display, one can more easily overlook the camera being in manual mode and think it is in auto mode.
7. I prefer OM handling, as I think shutter speed dial around lens mount is more intuitive and natural. However, many people would disagree with me.
OM Disadvantages/Nikon Advantages:
1. OM-2s frequently has a meaningful battery drain problem even when camera is not in use. Nikon FE is very good with batteries, if you make sure to turn meter off.
2. OM Zuiko lenses (except very common ones) are less common than many Nikkor/Nikon lenses.
3. If you upgrade to a Nikon FE2 or Nikon FM3a body, you get a far superior flash synch speed of 1/250 (and get TTL flash metering with dedicated Nikon flash), vs. OM being only 1/60. This higher synch speed makes those cameras far more useful for fill flash and freezing movement indoors. I recall Nikon FE has a flash synch of 1/125, which is better for fill flash, but no TTL flash metering.
4. Nikon allows you to use cheaper alkaline batteries, Olympus requires more expensive silver oxide batteries.
Both systems offer some outstanding lenses, with OM being generally more compact. The lenses of both systems can be easily adapted to Canon dslrs, but only Nikon can be used on Nikon dslrs.