If you are a national of a country in the EEA or Switzerland (and coming before 31 Dec 2020), or you have indefinite leave to remain, you don't need a visa to study in the UK.
If you are a national of any other country, you will require a visa for study. EU/EEA/Swiss nationals coming to the UK on or after 1 January 2021 will require a visa unless you have been granted settled or pre-settled status under the EU Settlement Scheme. Irish citizens do not require a visa because of the common travel area.
Types of visas
Most students will require a student visa (formerly known as a Tier 4 visa).
If you are coming to Oxford for a course or period of study that is six months or less in duration, you may be eligible for a Visitor Student visa.
For more information about the type of visa you will require to study at Oxford, visit the University Student Immigration pages.