OAK driver Guillaume Moreau has undergone emergency surgery after sustaining spinal injuries in an accident during Sunday's Le Mans test day.
The 29-year-old Frenchman will definitely miss the 24-hour race later this month, with a recovery period of at least several months expected.
An OAK team statement said: "Following a severe accident at the exit of the karting track corner towards the end of Sunday morning's session, Guillaume Moreau has suffered a serious but partial injury to his spinal cord's 12th vertebra.
"Guillaume has since undergone an emergency operation performed by Professor Olivier Lucas at the University Hospital of Angers' Neurosurgery Department. It was crucial to quickly reduce pressure around the spinal cord following the fracture.
"Although the operation went well, it will be several months before his level of recovery can be fully assessed.
"The thoughts of the entire team are with Guillaume and his family at this difficult time."
Moreau had been due to share OAK's #15 LMP1 Pescarolo-Judd with Bertrand Baguette and Dominik Kraihamer at the 24 Hours.