That's right; the reactionless drives in OA would only work if momentum is not conserved on a large scale, or if such a thing as
negative inertia were possible- both concepts are unlikely but not yet proved to be impossible.
The
Breakthrough Physics Program, initialy funded by NASA, came up with a lot of different reaction-free drive possibilities, some of which OA has adapted to suit our purposes; but each of these concepts requires the laws of physics to be subtly (or not-so-subtly) different to the physics currently known.