The motorway was closed in both directions for several hours following the collision, which happened on the northbound carriageway between junctions 10 and 11 shortly before 11.40pm on Saturday.
Police have confirmed a 60-year-old man from Oxfordshire and a 29-year-old man from Warwickshire died as a result of the crash.
Their families have been informed.
The man with serious leg injures was taken to hospital and three others were treated for minor injuries.
One driver said he saw about 30 emergency vehicles at the scene, near Cherwell Valley services.
South Central Ambulance Service confirmed five crews were sent.
The motorway was closed for several hours following the crash, but has since reopened.
Chief Inspector Henry Parsons has appealed for witnesses to come forward.
He said: “Our thoughts are with both men’s families at this difficult time.
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“We would like to speak to anyone who may have witnessed the collision who has not yet spoken to police.”
Anyone with information can call Thames Valley Police on 101, quoting reference number 1669 (23/12).