Matriculation is the ceremony which marks your formal admission to the University, at which academic dress is worn.
If your course starts in Michaelmas term, your matriculation ceremony will usually take place at the end of your first week. Visiting and Recognised students are not required to matriculate. For a full list of courses that do and do not have to matriculate, see the University web pages referenced below.
If you are a graduate who completed undergraduate study at Oxford, you do not need to matriculate again.
The Academic Office provides information about your matriculation in the Information for Undergraduates / Graduates pack that you will have received before arrival. They will also email you reminders about your ceremony before it takes place.
You can find out more about matriculation on the University web pages.