If you make an attack of opportunity, can you take an immediate action in the same round? It would make sense because immediate actions were invented after the PH (and therefore AoOs), but is this RAW?
I’m asking this question because in 5e (the system where immediate actions-or rather “reactons” are in the core rulebooks), an attack of opportunity is a reaction.
No, an attack of opportunity is not an immediate action.
An AoO is not an action at all, it's an AoO (If you look at the SRD under Actions in Combat, AoO are referenced but not listed). As such it has it's own rules for when they can be taken and how many you get (1/round, unless you have something like combat reflexes).
You can freely combine AoOs with immediate actions and any other kind of action you can take.