Yes, it might be OM — it's 360g.
I had an Olympus OM Zuiko E.Zuiko Auto T 135mm f/2.8. Compact with very good resolution. My Canon FD 135/2.8 has better contrast, though. Otherwise same optical performance.
Here's more about it:
http://www.mir.com.my/rb/photography/hardwares/classics/olympusom1n2/shared/zuiko/htmls/135mm.htm