This means you'd need to:
* Meet the incredibly high entry requirements for the specific course at Oxford. Your existing academic performance at Manchester would be crucial, but it's not guaranteed acceptance even with excellent grades. They'll assess your suitability for their program all over again.
* Secure an interview and potentially further assessments. This process is rigorous and competitive.
* Obtain a place on the course, which, as mentioned, is highly competitive, often even more so than initial undergraduate applications.
In short, while technically possible, it's a very challenging undertaking and requires exceptional academic performance and a compelling case for transferring. It's much more likely to succeed if you have a very strong reason for wanting to transfer (e.g., a specific research opportunity available only at Oxford) and exceptionally strong academic achievements.