JF: I have both an M42 to OM adapter and an OM to M42 adapter. You lose wide-open metering in either case, but it gives me flexibility. Recently I have used them together on an OM as a non-auto extension tube. OM to M42 adapter on the camera, into which is scrwed the M42 to OM adapter. Then a Zuiko lens. Works well.
Sorry I don't have spares to offer, but I believe mine were purchased from Spiratone, so you may be able to find the same or similar under another name.