There are many excellent 6th form colleges and schools who also have a 6th form in and around Oxford.
What it largely comes down to is what qualifications you wish to take, and how much you are able to pay.
State-funded OCVC offers 6th form courses, and The Cherwell School has a 6th Form.
Privately, the options are extensive:
St. Claires (offer international baccalaureate)
Schools with a 6th Form:
Magdalen College School
St. Edwards School
You may find that places are harder to get at short-notice from the schools, as opposed to the 6th Form colleges.
As far as a recommendation goes, all the privately-funded options are academically excellent. If pushed, I would recommend Magdalen College School and St Claires from the private selection and The Cherwell School from the state-funded.