What is the cheapest (used) OM-series SLR?
The OM-10, but there’s a catch. The original OM-10 had *only* automatic, not manual, exposure. To get manual shutter speed settings, you have to add the manual adapter, or get an OM-10FC, which has it built in. Still, an OM-10 costs only $50 to $70 used, and at the price, it can be a handy emergency backup camera.