Bit of both. Up to you really. The OAT DB is made available after your first progress test and obviously you can get Bristol access whenever you like. OATs MO however, is to teach you the syllabus, not just the answers to the exam questions (and please, I'm not arguing that no-one else does this before the anti-OAT brigade leap in!). If you don't demonstrate at least some understanding of the subjects throughout the "learning" phase, I doubt you'd be allowed to go forward for the exams.
This refers to the integrated course only, the modular route, both through OAT and other directions, differs.