To what extent are these cameras rugged against wind, rain, snow. Once I read a report on the OM4 which appeared to be very resistant to heat, shocks...
The OM4 and 4Ti havwe weatherproof seals around the major controls etc and are better protected than the earlier models. I once wrote off an OM-2n when it got wet, and the 2SP is not sealed either, so be careful with these.
What Richard said. I have never had a problem with my OM1/2s in inclement weather, but they are not weatherproof. I wouldn't shoot with them in a pouring rain unless I had the controls in something such as a ziplock bag. That said, the nice thing about the OM1 is that it's an all mechanical camera. Even without the meter, you can shoot at any shutter speed. I think the OM-2SP might be most at risk given the amount of electronics stuffed inside. The OM-2SP is more closely related to the OM-4 than the OM-2(n).
I've used mine (OM-1n & 2n) in all weather for over 20 years and never had a problem. That said, I DO take care to use an umbrella, etc. in the rain, and to not let it get covered in snow if the camera is still warm. Care is also needed in hot dusty/sandy environments. If you want a care-free camera for use in the rain, I'd suggest a Nikonos.
I once shooted with OM-3 in a rainy day. Didn't take long time (couple of minutes in "medium" rain) when the lightmeter stopped working -- apparently it didn't like the water. When the camera dried (it took a day) then the meter started working again.