It looks smaller than the OM and the olympus didn't have that square notch.From memory it looks like my old Olympus OM1n, I could be wrong though. Is it a Zuiko lens
I did look there and couldn't see it. The lens is a third party one, weighty and solid. I wanted to try it out on my Olympus digital but for the life of me cannot identify what adapter I need.Have you tried this...
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SLR Lens Mount Identification Guide by Richard Oleson - Vintagelens
for al the info on photography and early tech - SLR Lens Mount Identification Guide by Richard Oleson %vintagelens.nl

I realised I have an Olympus OM adapter so I tried it and it didn't fit. It's not an FD either though it almost fits my Canon A-1.From memory it looks like my old Olympus OM1n, I could be wrong though. Is it a Zuiko lens