An AB is an AB. When the job ships, it will be for day man or watch.
OT is not 'required' on union ships, but there is 'required' OT. Preparing to enter port, mooring, letting go, securing for sea and cargo is mandatory and an all hands evolution. Chipping, painting, general maintenance is voluntary OT.
6 ABs with 1 being the Bosun. All are assigned watches for when 2 ABs are required. (entering/leaving port, fog watch, reliefs, etc). Bosun is almost always 8-12