Originally off contract at the end of this season, O'Brien has signed a three-year extension.
Assuming he sees out the deal, O'Brien will overtake Michael Hagan as the longest-serving coach of the club.
The extension comes just days after the Knights fended off interest from rivals to lock in strike centre Bradman Best until the end of 2027.
O'Brien was under pressure to retain his job when Newcastle were struggling outside of the top eight midway through last season after finishing 14th in 2022.
But off the back of a Kalyn Ponga tidal wave and several unsung stars stepping up, the Knights were able to claim their first home final since 2006.
O'Brien was adamant after their finals exit that the Knights could not be content with that finish, and it had to springboard the club towards a more competitive future.
The new contract means he will have the chance to lead the Knights through that period.
O'Brien's deal means master mentors Wayne Bennett and Craig Bellamy are the only active coaches off contract for 2024.
Bennett has confirmed he will leave the Dolphins at the end of this season, while Bellamy's future has long been a year-by-year prospect at Melbourne.
10°C