The 81-year-old driver of a Fiesta suffered multiple injuries in the crash on the A4064 between Llangeinor and Brynmenyn on Saturday at 15:05 BST.
He is being treated at Cardiff's University Hospital of Wales. His 75-year-old passenger also needed hospital treatment for head injuries.
The Vauxhall Astra driver was taken to Princess of Wales Hospital, Bridgend.
He has been arrested on suspicion of causing serious injury by dangerous driving.
Police are appealing for witnesses.
A 24-year-old driver has been arrested after a crash which left two other men hurt, one seriously, in Bridgend.