I use an Olympus OM 50mm 1.8 adapted, which was quite nice when carrying the OM-1 travelling, as the lens could be used in both systems. Infact it is much better than the kit zoom or the Sigma 19mm I now have.
I would wish for focus peaking though. My adapter is a $10 from ebay and infinity is reached around the 10m mark, so the scale is wrong. Using magnification gives fantastic results but is inconvenient.