I have experienced a couple of OOAs and it really depends on the situation. Helpful eh?
One for example was a bolting diver on a check dive. Mask clearing and then doing the whole 9 yards, mask off, reg out and bicycling for their life to the surface. In that situation my purging reg was introduced fairly firmly in to their mouth. It was a reaction and after thinking about it a lot, I think it was the right one as there was no way this guy was thinking to purge a donated reg after he had just spat his own out. Maybe this doesn't qualify as a 'real' OOA but I include it as a response to the ' to purge or not to purge' question. Again, my answer is: it depends.
Another OOA was signalling OOA and I swam to him with the octopus extended. His tank was turned off. (Father and son Master Scuba Divers from the UK) I signalled to the father to come over, and then turned his air on as well
I myself ran out of air. First noticed it was difficult to breathe, weighed up options, found restaurant staff diver in another group, by that stage I was OOA. Signalled and grabbed his alternate.
I have never had someone rip the primary from my mouth, or seen it happen during a dive, or worked with anyone else who has had this experience. It
could well happen, but IMO it seems unlikely enough to happen to justify the inter-agency bickering that can go on sometimes.
If someone did rip my primary out, I hope I would have the presence of mind to take my octopus. If the OOA diver firstly stole my reg, then managed to wrap both my arms together... well, there may well be a bit of struggling going on, potentially a raised knee to the goolies or a Liverpool kiss? (Joke... but serious- gotta do what ya gotta do)