O'Sullivan was Cleary's understudy during the 2022 season at the Penrith Panthers, playing 11 first grade games - more than he ever would have expected.
It started with a bang as O'Sullivan guided Penrith to an excellent start to the season while Cleary was out injured, continued during the State of Origin period, and then again at the end of the season after Cleary was suspended for a devastating lifting tackle on Dylan Brown.
Cleary's return in time for the finals would push Penrith to their second-straight premiership, but O'Sullivan's impressive form ultimately saw him picked up by the Dolphins.
The signing, which was rumoured for some time, came after Wayne Bennett had claimed the club had made no contact with O'Sullivan, who is also the son of recruitment boss Peter O'Sullivan.
Speaking to AAP, Sean O'Sullivan said he learned how to be comfortable from Cleary.
"I have realised who Sean O'Sullivan is," he told AAP.
"The foremost thing I learned from Nathan is being comfortable with who you are and being able to play your own football.
"I know Wayne doesn't want me to be anyone but me. I am trying to prove to him that I am the man for this job."
O'Sullivan said he would be forever in Cleary's debt for the confidence he was able to gain out of his single season back at his junior club.
"When you are comfortable in your own skin like Nathan is you are walking towards those big moments," O'Sullivan said.
"I am forever in his debt for the confidence he gave me and what he taught me about footy and how to build a game in eighty minutes. I will be taking those lessons into the coming season that is for sure.
"Penrith didn't want me to be Nathan. They just wanted me to be Sean.
"As a result I realise what my strengths are and just have to nail my own role. I know the other sixteen players in the team will be doing theirs too. That is what we are looking to build here at the Dolphins."
Playing a single game for the Roosters, O'Sullivan would move to the Brisbane Broncos, where he played eight games over a two-year period, before managing 12 games at the Warriors in 2021.
Despite being a second option, O'Sullivan's 2022 at the Panthers was the best of his career, with his form at reserve grade level guiding Penrith to the NSW Cup and State Challenge crown away from the NRL.
It's form he will now need to desperately replicate at the Dolphins, with the club being widely predicted to endure a difficult first season as the NRL's 17th club.