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.

The principal of Saint Ignatius College in the northern Sydney suburb of Riverview has informed former and current pupils of the allegations.
Paul Hine said in his letter that the allegations concerned "child sexual abuse over 30 years ago".
The Roman Catholic college is among the country's top private schools.
Mr Hine said the college was assisting New South Wales police with the complaint, which was made to the standards office of the Australian Province of the Society of Jesuits, the owners of the school.
In a statement, Mr Hine said: "The College and the Jesuits wish to express, in the strongest possible terms, their abhorrence of sexual misconduct or any form of abuse involving children.
"We wish to respond to past abuse and suffering with a commitment to healing, openness and accountability."
Mr Hine also encouraged any person who "believed they were harmed" while in the care of the school to get in touch.
As well as Mr Abbott, the school was attended by Barnaby Joyce, the minister for agriculture, and Anthony Fisher, who is archbishop of Sydney.
In 1997, Peter Bohrsmann, a 57-year-old teacher at the school, took his own life on school grounds after he was accused of sexual abuse. Bohrsmann had denied the allegations.
Australia's Royal Commission is currently investigating allegations of child sexual abuse at a variety of institutions including schools, churches, and orphanages.
The Commission's inquiry was launched by former Prime Minister Julia Gillard in 2013.

Captain Jerry Kyd hailed a "truly seminal moment for Scotland" as he took command of the future flagship at Rosyth Dockyard in Fife.
The 65,000 tonne ship will be based at Portsmouth Naval Base
HMS Queen Elizabeth is one of the largest surface warships ever constructed for the UK.
A second carrier, HMS Prince of Wales, is still under construction in Rosyth.
Captain Kyd said the Queen Elizabeth will be tasked with "protecting British maritime power around the world wherever the government needs it".
He said maritime warfare has "changed very little" since the Battle of Jutland, 100 years ago next week, when Royal Navy battle cruisers set out to enforce a British blockade during the First  World War.
But the Queen Elizabeth will be equipped to deal with future threats, including unmanned aircraft, robotics and cyber attacks.
He added: "As a naval officer I always want more ships and more aircraft carriers, but I can't see in the foreseeable future two more ships of this scale being built in the United Kingdom".
A 24-year-old driver has been arrested after a crash which left two other men hurt, one seriously, in Bridgend.
[NEXT_CONCEPT]
Police in Australia have received allegations of sexual abuse at a school attended by Australian Prime Minister Tony Abbott.
[NEXT_CONCEPT]
The new commanding officer of HMS Queen Elizabeth has taken control of what he expects to be one of the last major aircraft carriers built in the UK.