I also realized that you cannot change the screens for the FM but I believe you can for the MX and I know you can for the OM's and so I think that I am leaning towards one of those now.
The MX can take numerous different screens. It also takes the LX screens, which are brighter. The extra brightness means the meter might underexpose up to 1/3 stop. Not a problem for me, though the meter can be easily adjusted by a service shop to compensate.
Is the MX and OM with their respective compact lenses (40mm f2.0 & 50mm f1.8 for Olympus and 40mm f2.8 & 50mm f1.7 for Pentax) in the same league in terms of size? More precisely: MX + 40mm vs OM + 40mm and MX + 50mm vs OM + 50mm. Both cameras/lenses seem very suitable, in terms of size. From the picture provided by Les Sarile, it seems so, but I'm unsure which lenses those are.
I don't know about the 40mm lenses, but IMO with the 50mm lenses they are definitely in the same league.
Focus and viewfinder is important to me too.
Is it practical to focus with any of these combinations with an all-matte screen? I don't like focus-aids which is one reason why I tend to not use 35mm SLR's.
I use an all-matte LX screen in one of my MX's and a matte/microprism dot LX screen in the other. And my LX and Nikon F3HP both have all-matte screens. I find for me, focusing aids are only needed in dim light.
The OM-1 and MX cameras feel different and have some differences in features. I like both better than the FM series because I find the FM finder "squinty" and hard to use with glasses.
The MX has a more traditional layout, with the shutter speed knob on top, aperture ring at the rear of the lens, and DoF preview lever on the front. F/stop and shutter speed are both visible in the finder. Meter indication is by LED. The viewfinder magnification is somewhat more than the OM-1's though the OM-1's is (I think) next largest, and they both show almost the entire picture area. The shutter knob on MX's is heavily detented, making it harder to turn than on most cameras. Once used to it, it's not that a big deal to me or others I've talked to about it.
The OM-1 has the shutter speed ring on the front around the lens mount and the aperture ring in front of the focusing ring on prime lenses. DoF preview is by a button on each lens. Meter indication is by needle, and f/stop and shutter speed are not visible in the finder.
Subjectively, this is my take on each. I would gladly use either. They are both great machines. Both are very quiet and have very low vibration, especially on first curtain opening (when it really counts).
I chose Pentax because of the LX. It's a beautifully made machine about the same size and weight of a Nikon FM/FE, but with excellent, bright and clear interchangeable finders and is almost as easy to use with glasses as the F3HP. I also have two MX's and two ME Supers and what stands out for me with all is compactness and great viewfinders. Another reason, not to be discounted, is that I had always used lenses with controls that turned the same way (Fujica ST, Pentax Takumar, Nikon). I didn't have to retrain myself after four decades, especially as I still use some Nikon stuff.
My point is that I chose based on aggregate reasons. If I were looking for what you're looking for, it's hard for me to say if I would have settled on the MX or the OM-1.
The unusual control locations on the OM-1 make it for me an ergonomic delight. Some people don't like it, but I find for me it makes the camera very easy to use, handling as if it's larger than it is. I find the viewfinder very easy on the eyes, even with glasses, though as with the MX, it's so big it's hard to take it all in at a glance. The sensation with both is sort of like sitting a little close to a movie screen. But there's something about the OM finder that feels a little better to my eye than the MX. On the other hand, it has a little less contrast than the MX, just not quite as crisp, so it can be a little harder to find perfect focus without an aid. The lack of information in the OM finder would be a problem for me sometimes, especially with hand-held macro where I am often changing both shutter and aperture settings. The original meter battery is a mercury type, so you will have either use the air-cell type battery or have it adjusted to take the 1.5V silver oxide battery (what I would do). Or if you get lucky, it will have already been adjusted.
The OM-1n is the newest model and IMO, the one to get. It has a flash ready light in the finder, and supposedly Olympus made some internal changes for improved function and durability. Regarding flash on camera, the hot shoe on the OM-1 is detachable, and it has a tendency to crack or break. The MX hot shoe is permanent and quite durable.
If you could handle each one, that might decide it for you right there. They are both really good. Even with lenses-my Pentax M-series lenses are small, light, and smooth. The OM lenses are the same, but also (to me) have a jewel-like quality to them that is obviously subjective. On the other hand, when Olympus went to plastic for some of their aperture rings, the rings lost their precision feel for me, feeling cheap and no longer nearly as smooth.
What really matters is which combination of features and feel works for you. If you can get to a photographic swap meet in your area (there's a couple a month here in SoCal) then you would have an opportunity to handle each and decide. Or, given prices these days, you could buy both and just sell the one you don't want.
Here is something interesting to illustrate what I'm saying: I never had any interest in the ME Super. I don't use AE and I didn't like the push-button shutter changing when I tried it on the Pentax 645. But one day a friend needing money offered to sell me his ME Super. I bought it to help him out more than to have the camera. My first impression was that it had a nice viewfinder and felt good in my hands. I set it aside and didn't even pick it up for a couple of years. One day I decided to sell it, and went to clean it up. I could see it needed new seals and probably a CLA, so I figured I'd sell it cheap. But while I was messing with it I started realizing what a good job Pentax had done on the camera. The viewfinder was very bright, clear and contrasty, it felt good in my hands even though it was tiny, it had a nice balance and feel. It just had a charm to it. So I decided to get it CLA'd ($63), and to use it for a while even though its non-interchangeable screen had the split-image/microprism and I didn't like the push-button shutter change. I found upon using it that once I figured out how to best use the push buttons they ceased to be an issue, and I can now change speeds quickly and precisely. I use the focusing aids because they are there even though I prefer plain matte. It's become my usual walking around coat-pocket camera with a 28 or the tiny Pentax 24-35 zoom, plus a 50 and sometimes something longer.
So until you handle something you'll never know for sure if you'll like it. Specs and features are not the whole story.