It closed part of the southbound carriageway for an entire day.
Thousands of drivers were affected after a big diesel spill, around 500 litres, last Friday.
Fire, Police and Highways officials were called to the scene.
No one was hurt, but the road needed resurfacing.
The incident caused miles of tailbacks, with traffic having to be turned around from the back of the queue.
Police now want to speak to people who saw what happened just before the crash at around 6:30am.
Investigating officer PC Kevin Crick from Roads Policing, said: “I would like to speak to anyone who witnessed the collision or who saw the lorries and a dark-coloured Audi just prior to the collision.
“If you saw what happened or have any dash-cam footage of the incident, please contact police.”
The crash happened between the junctions for Peartree and Wytham, with an Audi also involved.
Anyone with any information should contact Thames Valley Police on 101, quoting the reference 43190004823.
?APPEAL?— TVP Roads Policing (@tvprp) January 9, 2019
Officers are appealing for witnesses following a collision on the A34.
It happened at 6.30am on Friday on the southbound carriageway between Peartree and Wytham.
Two lorries and an Audi were involved in the collision. No one was injured.https://t.co/Tk4qSzAwlR