The RAH is a beautiful theatre, round with a lot of charm. Even it's big is still a theatre.
The O2 is a big hangar. Too big, excessively big, with no soul, not beauty, not charm. Just a big hangar.
I've been in both watching MK and O2 totally sucks versus the RAH, which is probably one of the best and most beautiful venues not in London but in the world.