A CNS tox hit has two stages, tonic, where the body goes in to spasm and clonic where the body relaxes. During the tonic phase, the airway will be blocked, so ascending the diver at that stage would probably lead to baura traumas. Once the diver goes to clonic phase the airway will reopen, it would then be safe to surface.

If a diver gets a CNS hit then they probably ingest water, and could also suffer some form of baura trauma, in either case oxygen would be the treatment of choice.

CNS tox will generally come from increased PO2 levels, however it can also occur through long exposure to O2. Increased pressure will increase the rate at which this can occur, however this can still happen on the surface (happened to a friend of mine a few weeks back...). It's not really an issue though, getting a CNS tox hit on the surface, whilst undesirable, is not going to cause any long lasting problems. It's also rare, and would require a very long exposure, not something that should be considered when providing O2.