.....between Marylebone & Bicester North only, and between 8am & 8pm only.
Timetable is
hereAll normal off-peak fares available all weekend available on this day too. Not sure about advance fares, but tickets are available on their booking engine, so go & check if you are interested.
I'm not sure Advances are generally available between those stations, so I'm doubtful. But Groupsave definitely is.