Following a fault with the signalling system between Banbury and Bicester North trains are running at a reduced speed on the line towards London.
Train services running through these stations may be delayed by up to 30 minutes.
Disruption is expected until 5pm, according to Chiltern Railways.
Chiltern Railways has apologised for the disruption.
A statement said: “Apologies, due to a fault with the signalling system between Banbury and Bicester North trains have to run at reduced speed on the line towards London.
“Train services running through these stations may be delayed by up to 30 minutes.
“Disruption is expected until 5pm.