Flash recommendation for Olympus OM2S

[Clemmie]

Well, the OM-2S ain't exactly a spring chicken (sold 1984-1988, according to a quick search - though I believe they predate that by a few years), and flash mounts have certainly become more proprietary in the past several years, so there is valid cause for concern.

I found this 'Olympus Archive' site, which may have more detailed information for you:
http://www.heier.com/olydocs/

One of the documents available there, is the original manual for the OM-2S - which shows Olympus flash units T20 and T32 for 'on top' hot-shoe mounting, and the T45 side-grip unit (styled not unlike a contemporary Metz pro flash).
You might also want to look into 'time proven' flash units made by others in the same era as the camera - such as the Vivitar 283.