Hi.
I'm sorry to learn about your over-exposure problem. I usually take pictures in bright sunlight using a hand-held light meter. So far, (touch wood) no problems. I don't take night pictures 'cos I don't know how to, so I can't comment. There's a reciprocity law failure with using film for night photography - others who know would have to advise you on that.
Few things I can think of/suggest:
1. Since the OM-1 is a fully mechanical camera, make sure all settings are set correctly, including ISO/ASA speed.
2. As Teo mentioned above, check that you're using the right type of battery.
3. Use your DSLR as a light-meter, and make/compare your settings based on that.
4. Check if your lens aperture is closing/opening as it should to all the set apertures when you press the shutter. (I find it easier if I open the back cover when not loaded with film, use the B setting to keep the shutter open, and look through the lens from behind.)
5. Check to see if your shutter is opening/closing as it should. Different settings should give different speeds.
6. Make sure you close the back cover securely after you've loaded your film.
7. Check if the light seals/foam around the edges of the back cover are in order - there might be light leaks.
Any suggestions from others?