Getting 2’s and 3’s can mean many things- but none of them suggest that Oxford is the right place for you (for undergrad). It could mean a lousy teacher- no fault of yours. It could mean poor study habits. It could mean that you learned some of the material but not all. It could mean that you learned most of the material but are not a strong writer or didn’t do well on the actual calculations.
None of these reasons bode well for Oxford. Your classmates will have thrived in rigorous environments and likely did well in everything. So whether or not a 3 in AP Bio will keep you out-- you’ll be at university with students who excel at everything. The pace of work is just relentless- there will never be a moment during term when you don’t have something you are working on, should be working on, or need to start because it’s due ASAP. It’s not like at a US university where the “shopping period” at the beginning of the semester is a chance to get your sea legs.
Where do you live? So many great colleges in the world!