As described in a comment above, I also dug up the old Olympus OM goals of my parents, and envy I immediately took to try them.
This ring is screwed without problems in my GX1 and Olympus OM 100mm to 400mm 2.8 and 6.3. No games, no blocking. Good necessarily requires manual focus, it's weird at first, especially with 400mm weighs a dead donkey and a half (compared to a 45-200mm Panasonic, but you have to compare what is comparable.
Finally: good OM adapter ring >>> m4 / 3 does his job.