"The section between Oxford & Bicester London Road was reopened on 15.5.1989 and in 2001 the Strategic Rail Authority looked into reopening the remaining part of the line for passengers between Bicester and Bletchley but this proposal has now been rejected."
From the First Great Western Website :
17:44 Oxford to Bicester Town due 18:10
This train has been cancelled. This is due to a member of train crew being unavailable.
Road transport will be provided.
18:24 Bicester Town to Oxford due 18:50
This train has been cancelled. This is due to a member of train crew being unavailable.
Road transport will be provided.
18:59 Oxford to Bicester Town due 19:25
This train has been cancelled. This is due to a member of train crew being unavailable.
Road transport will be provided.
19:30 Bicester Town to London Paddington due 20:59
This train will be started from Oxford. It will no longer call at: Bicester Town and Islip. This is due to a member of train crew being unavailable.
Road transport will be provided between Bicester and Oxford, calling at Islip.
Those are the "key commuter" Oxford - Bicester Town services and the 19:30 Bicester Town to London Paddington is the last train of the day.