I don’t think you’ve ever mentioned Olly screaming before, so this must have been a shock.

Screaming comes from a really, high intensity fear. I don’t know what literature may be behind rabbits and whether they dream, but I suspect if it was a bad dream scream, it shouldn’t happen again, as those would have to be quite rare, or else we’d have a lot more instances of people reporting their rabbits screaming while sleeping.

I would continue to monitor and if he does scream again, assess how the situation relates to this particular and see if you have draw any commonalities as to the root cause. If it ends up being something Olly does more frequently, I would recommend taking him to a rabbit vet to have him checked out, as it should not happen each time he sleeps/just frequently in general. Poor Olly.