The 26-year-old was the foundation Dolphins No.7 and was in career-best form until a bicep rupture in round four of the 2023 season.
He returned to the position for the final 10 rounds with mixed success, but Kodi Nikorima and Isaiya Katoa emerged in 2024 as the preferred halves combination.
O'Sullivan is off contract at the end of this season and wants to stay. Aside from being settled off the field with a young family, he has rejected overtures to leave as a matter of pride.
"I love it here at the Dolphins. I am a foundation player here and we are building something really good," he said.
"Maybe I could have taken some easy options, but I'm glad I didn't.
Sean O'Sullivan has rejected overtures from other clubs to stay with the Dolphins. (Jono Searle/AAP PHOTOS)
"Whoever plays seven and six is going to earn it through their pre-season and trials."
O'Sullivan is yet to establish himself as a regular NRL playmaker through 56 top-grade games at five clubs.
His time to shine is now, but the pressure has not worn him down.
"You go through some highs and lows. You don't want to go through the lows, but at the end of the day you learn from it and come through a better person and player," O'Sullivan said.
"For me it is about learning my craft and what I can bring to a team and bringing that every single day ... and especially on game day. That is really important for me and something to continually chase and work hard at."
O'Sullivan is an organising half who likes to kick to corners and control play rather than showcase a running game in the fashion of Nikorima, who has also played as a fullback.
Tongan international Katoa is the first-choice Dolphins half and has a kicking game and organisational skills that will make it tough for O'Sullivan to play alongside him.
"I think the way we play, the five-eighth does play a bit differently. I will just worry about what I bring to the team, and whatever position that is I will do that," O'Sullivan said.
"Seven and six is played differently at the Dolphins to some other teams."The seven does do a lot of the kicking, but if me and Izzy did play together we could definitely share that role. It is about what a person's strengths are, and you have just got to accommodate that."
As for his future beyond 2025, O'Sullivan said it was all about short-term gains for now.
"I have had a couple of chats to my manager and I am just trying to focus on what is going to help me in the near future, and that is trying to train good and nail these trial games," he said.
The Dolphins have trials against North Queensland and Gold Coast in the NRL Pre-Season Challenge next month.
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