You can almost take it for granted that an Oxbridge applicant will have the relevant predicted grades and work experience. As for the actual college, if you decide to indicate a preference, make sure you have done some research and ideally visited the university, as you are highly likely to be asked about your choice in the interview.
The main form of assessment will be the interview, and you may have more than one. As well as the ‘standard’ domains you can be asked about like most other medical schools, expect some questions that expect you to apply what you have leant in your A-level studies to a scenario. Examples include: -
How would you work out the amount of water in a swimming pool?
Tell me a place in the human body where you would find the benzene ring?
There is a fridge in a closed room. The door is opened. What happens to the temperature in the room?
Good luck in your application!