From main Alty site:
Oxford United have announced as usual that for our game at their ground (31st October) "Tickets are cheaper if brought before the day. The prices in the North stand (along the side of the pitch) where your fans will sit are as follows:
Adults: £15 (£17 on the day)
Students with NUS Card: £11.50 (£13 on the day)
Over 65: £9.00 (£10 on the day)
Under 16: £9.00 (£10 on the day)
Under 7: FREE (FREE on the day)
Tickets can be bought in advance from Oxford by ringing 01865 337533 or from the website but a booking fee of £2.00 for posting and £1.50 collection is added per booking.
So where is the the incentive to get tickets early?
Maybe you can reserve an under 7s ticket and then get it delivered for s quid as well