One of the key criteria for Oxbridge admission is the academic performance. Usually, they ask for at least A*, A*, A. They also assess your forward thinking and logical and analytical skills.
Cambridge University : They ask for a Supplementary Application Questionnaire (SAQ) other than UCAS form. They also conduct subject specific tests either prior to or during interview. BMAT and UCAT exams are sat before interviews.
Oxford University : They also have a separate test. Also, written course work is asked for.
At both universities interviews are extremely important which may be conducted more than once.
Oxford has 39 colleges whereas Cambridge has 31 colleges.
Normally one should apply for a specific college but you can make an open application so it will read as.