The winner of Knights-Dolphins is guaranteed a spot in the NRL finals, but if it ends in a draw that honour will go to whoever is victorious out of Saturday's encounter between Stuart's Raiders and Flanagan's Dragons.
After the Knights and the Dolphins both scored big wins in their penultimate-round wins against Gold Coast and Brisbane, respectively, O'Brien thinks Sunday's game will go down to the wire.
"I think it'll be much tighter," he said.
"Well, that's the way we're preparing for it. If it takes 90 minutes, then so be it. We're in for the long haul."
Dane Gagai's kicking game was all that separated the two sides in their earlier meeting this season, with both teams scoring three tries apiece in the Knights' 18-14 triumph.
Newcastle have the home-ground advantage but there are scant clues to pick a favourite.
Both sides sit on 28 competition points after 11 wins and 12 losses, with the Dolphins' superior for-and-against giving them a momentary edge on the ladder.
Given what's on the line, the sell-out McDonald Jones Stadium crowd could be set for a thrilling finish.
"It'll be two teams very similar in styles going at it," O'Brien said.Â
"Both teams like a high completion rate game. It'd be good to be sitting on the hill on Sunday afternoon watching these two teams go at it for a result that's really important to both clubs."
The 46-year-old is looking for another powerful performance from second-rower Brodie Jones, who set up a try and made 115 metres with the ball against the Titans.
O'Brien scuppered speculation the off-contract Hunter junior could be squeezed out of the Newcastle set-up because of an overabundance of forward talent.
"Absolutely," O'Brien said when asked if Jones would be at the club next year.
"He's done a hell of a job. Local boy, come through our system, developed him. Yeah, he's someone that we're really proud of with where he's got to on his journey, and I know that he would want to stay as well."
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