Well, most likely not. Maybe they'd do that occasionally with 12 A*/AAAA people for whom AAB is just a clear bad exam or a slip, but I doubt they would in the majority of cases.
To put them down as an insurance wold be extremely risky, as it's AAA or (probably) no uni at all this year. My friend did the same actually with his Maths app., his firm was AAA + Step for Cambridge, and insurance was AAA for Imperial; he was so confident he'd get at least AAA in his exams. He failed his Step and got AABB; Cambridge wouldn't let him in. It took Imperial at least a week to decide to actually let him in anyway, but that week was the scariest of his life, and he came extremely close to having no uni at all to go to. So I would say that it's probably not worth it. You won't really find a much better uni that'd take you on AAB anyway, bar Bristol and KCL, who only offer it to AAA applicants anyway.
Congratulations on your Oxford offer, best of luck getting it!!!