There are five Permanent Private Halls (PPHs) at Oxford admitting undergraduates.
PPHs tend to be smaller than colleges, and offer fewer subjects. (See Which PPHs offer my course?) However, students at PPHs are members of the University just like students at colleges, and have access to exactly the same University facilities and activities. Regent’s Park is the largest PPH, and accepts applications from men and women of any age. St Benet’s only accepts male students. Blackfriars, St Stephen’s House and Wycliffe Hall only accept mature students. ("Mature student" means 21 or over at the start of October in your first year.)