A*AAB, for an Oxford applicant, is kinda low. Sure, it'd make their offer, but you're going to have a hell of a lot of A2 applicants with 3/4 A*. Worth a try, but you'll probably be wanting something closer to A*A*AA or more.
Don't take more A levels. 4 is fine and critical thinking is a waste of time (typing this with the critical thinking teacher five feet away, better hope she doesn't look over my shoulder...)
Oxford care a lot about GCSEs.