Personally, I would avoid both the OM-3 and OM-4 because of the battery drain issue. They're good cameras but they do suddenly stop working properly at inopportune times. The 'ti' versions are a considerable improvement. I also wouldn't bother with Zuiko zooms which, in my experience, were of mediocre quality.
The OM-1, OM-2 (and OM-1n and OM2n), OM-3ti and OM-4ti are very good indeed. So are most of the prime lenses which are still very affordable.
Accessories like dedicated flashguns and motordrives/winders are also cheap and good. The OM Motordrive 2 is by some margin the best add-on motor drive I've used: I have three and they're great!