I'd recommend sticking with Olympus Zuiko glass. And I kinda love the hell outta my Zuiko 50/1.2, but these days, it probably costs about twice what I got it for, if you can even find it. The 18/3.5 and 21/2 are probably the most-sought collectors' prizes for full-frame digital shooters. If you want a portrait/telephoto lens, I'd say looking for a 135/2.8 is probably the easiest/cheapest.
The good news is Olympus OM lenses have a great reputation optically, and there was a pretty wide selection to choose from. The mir site can give you an overview of what OM lenses were made as well as the nomenclature. And this fredmiranda forum's Alt. Gear board thread might help with examples or experiences with a specific lens.
The bad news is, unlike Minolta MD/MD or Canon FL/FD, OM lenses are useful on dSLRs are well as mirrorless. Canon shooters can adapt OM lenses with simple rings to their dSLRs just like mirrorless shooters can; and Leitax makes kits for Nikon and Pentax. So you're competing with a pretty large pool of other folks to snag those nice little OM lenses. So, yes, these are rarer and more expensive on the used market than they were.
In the US, KEH, B&H, and Adorama would be my first stops to look for used OM glass, because their grading is reliable and they have return policies. You can also try eBay, but you need to make sure the seller has a return policy, or that you're really confident you can accurately grade/judge the lens's function from the listing. And that you have a good affordable CLA guy. :)