The proper name of the X90 coach service between Oxford and London, the Oxford Espress is run by the Oxford Bus Company and is a natural competitor to the Oxford Tube. Prices are similar, though last I checked the Espress was marginally cheaper. Previously the coaches were less comfortable, but with the new name came a new fleet of coaches with more leg room and a toilet. Espress coaches are single decker rather than the Tube's twin decks, but for some reason it is less popular and hence less crowded.
The X90 picks up from the Bus Station, on High Street and St. Clement's and at [Thornhill Park and Ride]?. In London, it drops off at Hillingdon, then Baker Street, Marble Arch and Victoria Coach Station.
When Oxford Bus Company's parent group (The Go-Ahead Group) also owned the Oxford to London rail franchise, night-time Oxford Espress services would accept rail tickets after the trains had finished for the night, but the rail franchise ended in 2004, and this deal no longer exists.