O rings come in various thicknesses; like 1mm, 1.5mm, 2mm, etc. if the atomizer won't fire, after snugging it down to the o ring, it's because the positive post on the atomizer hasn't reached the point to where it's contacting the positive connection in the Reo. Use a thinner o ring, and it should.
For me, the purpose of the o ring is twofold. First; it allows 'indexing' the atomizer, so that the air inlets on the atomizer are where I want them (the new 510 connector eliminates this need). Second, if I should happen to drop my Reo, the o ring under the atomizer mitigates the stress on the 510 connections, should the atomizer hit the floor first. Without the o ring, it could bend or break the 510 stub on the atomizer.