Having left the field in Friday night's game against the Broncos, O'Sullivan was clearly in pain shown by his facial expressions.
The pectoral injury means he will be set to miss between 12 to 16 weeks and will not return until the end of June at the earliest.
O'Sullivan has been a shining light for the Dolphins and has been able to lead the team in attack. As he faces a stint in the injury ward, Anthony Milford will have to step up in his absence
So far this season, O'Sullivan has set up three tries, two line-breaks and forced two dropouts. He has also been effective in defence with 93.1% tackle efficiency.
Tesi Niu will also join him on the sidelines after he damaged a medial ligament in his knee on Friday night.
Bennett will also be able to select Edrick Lee who has fully returned from injury and played in the Hostplus Cup last weekend.