There's miles of tailbacks on the Southbound carriageway this morning.
Fire, Police and Highways officials were called to the scene between the A44 Peartree Interchange and the A420 Botley Interchange.
There's reports of hour-long delays.
Highways England says emergency resurfacing work is being carried out and there's a 500 litre diesel spillage to clean up, which will 'take several hours.'
The Thames Valley Roads Policing Unit says no one has been injured.
Highways teams are working to get cars stuck in the closure, out, by u-turning traffic from back of queue.
Other drivers are being urged to use the Eastern bypass as a diversion route.
Stagecoach Oxford tweeted to say its S5 service is delayed.
UPDATE: Please be patient if you're in the closure on #A34 south between #Peartree and #Botley nr #Oxford. There's lots of HGVs and cars to clear and it's nose-to-tail. We're doing what we can but the limited space of road means it'll take a while to get everyone out.— Highways England (@HighwaysEAST) 4 January 2019
Caught in delay after accident and diesel spillage on A34 - been here an hour. Send lavatory, coffee and bacon sandwiches please.— Andrew (@ChaplainPemb) 4 January 2019