The A34 was closed in both directions following a "medical emergency".
The road was shut both ways earlier this evening between the Marcham Road interchange at Abingdon and Hinksey Hill at Oxford.
A spokesman for South Central Ambulance Service confirmed they were called out to Bagley Wood Road in Kennington, where the bridge goes over the A34, at around 6.50pm.
An ambulance attended a medical emergency on the bridge, but the spokesman would not provide more details on what kind.
They did confirm the incident involved one person and that no-one was taken to hospital.
The road was reopened in both directions at around 7.50pm.
Oxford Bus Company traffic confirmed the reopening at 7.55pm on their Facebook page but said traffic was still heavy in the area.
The bus company said it was advised the incident - the nature of which has not been confirmed by police yet - was at the Abingdon North sliproad.
The Highways Agency said the closure was a police matter but was due to someone being seen on a bridge over the busy trunk route.
No-one from Thames Valley Police was available for comment.