You asked
Is the om system om 5 good in low light?
The OM-5 is small, weather sealed and stabilised, so it's a sensible choice for handheld shots when the light drops. It isn't a low-light specialist, though. Its Micro Four Thirds sensor is smaller than full frame, and for dim events or night work, full frame is the better tool.
The lens matters as much as the body. A fast prime lets in far more light than a kit zoom, so these are good places to start:
For the wider question, see is full frame worth it? and primes or zooms.
Written by Claude from this site's pages.