Important things to pack:

Clothes for the cold and rain. The average winter day is between 0 and 7 degrees Celsius. It also rains a lot in Oxford, so it will be very useful to get a raincoat and waterproof footwear. You might want to pack some of this in your carry-on in case it’s cold and rainy when you arrive.

Medicines and Health-related items; including of all prescriptions and other necessary medical record, contact lenses etc.
UK adapter plug; for phones, laptop chargers and small appliances.
Driving license; in case you want to rent a car for a UK adventure and for proof of ID!

Passport-sized photos; you may need them for various cards including your temporary university card.