Best cameras built ever by Olympus. After all those years they still shine. It is extremely joyfull to use them and they are etremely reliable.

Here are the things you need to be aware of while buying one:

1- FILM WIND SLIPS: This happens when the mechanism goes bad due to force and over usage, there is a potential fix which needs professional handling, but it does not work always so the part mneeds to be replaced. Either way you are looking at least $100-150 to get this fixed, because it is a time taking fix which needs quite a bit disassembly. 

2-THERE IS BLACKNESS AT THE BOTTOM OF VIEWFINDER: This is due to some internal seals goes bad and destroys the prism's coating. No fix other than replacing it.

3- OM2 or OM1 METER DOES NOT WORK: This is a tricky issue, it might be as easy as one loose connection or cold solder to battery contacts, or as difficult as the the lever mechanism that sets the aperture setting for the metering is broken. When you buy one, do not assume you can fix it no time. Because, to find the cause of the problem before you buy is extremely difficult. If the lever mechanism is broken, you are looking nearly full disassambly plus this mechanism is quite complicated. I would say do not bother. The soldering problem can be at several places, might need quite disassembly.

4- BAD LIGHT SEALS: you camera light seals needs to be replaced at this age. If it not already done, you are looking around $50-75 range to have this fixed.

5- OM10 ALWAYS FIRES AT THE SAME SPEED: I have seen this on couple OM10 cameras, but I would thing it could happen for OMG as well (since they are very similar.) As you might know, OM10 fires at 1/100 sec always regardsless of whatever the light meters says if you do have a lens mounted to the camera. This symptom looks like due toof ot the pins in the mount assembly is not engaging and camera stil thinks that there is no lens mounted event though the lens is already mounted.

Therefore, my suggestion is buy from a good reputation seller or ensure you check all those once you get it and you have option to return.