O'Brien has also stuck by the decision to drop Greg Marzhew, bringing Enari Tuala into the team, while suggesting the club are keen to him in the Hunter, despite the fact he has only played a single game in first-grade to date this year.
Marzhew, who was reportedly dropped for missing the team bus, will have to battle for his spot back in first-grade.
O'Brien said the form of Tuala in reserve grade was the key catalyst behind making the decision easier, while his first-grade form previously has been strong, with 32 tries scored across his previous three seasons in the Hunter.
"Definitely. He [Tuala] has played really well in the Cup, I'll grant Enari that. As hard as the decision was with Greg, it made it a little bit easier that Enari has come in and done a good job for us in the past. He is Mr. Dependable, he is durable. I know the club are keen to keep Enari here," O'Brien said.
"Importantly, I don't know if that's his number one focus about the contract extension, he just wants to come into the team and perform his role, and everyone is excited to have him in there.
"The easy decision was to play Greg, but it wouldn't have been the right one, so Enari coming in has made that a little bit easier. That's what we want. We want the guys to be competitive and have the next man up mentality."
Saifiti will be replaced in the side by Jacob Saifiti for this weekend's game, while O'Brien confirmed Fitzgibbon has made it through the week and is fit to play.
Fitzgibbon, who has made 12 appearances this year, hasn't played since injuring himself in Round 13 against the Manly Sea Eagles, but his return - straight into the starting side - will come as a welcome relief for the departe Knights after he scored three tries and was a threat with the ball early in the season.
"He has ticked everything this week, trained this morning. He is good to go," O'Brien said on the return of Lachlan Fitzgibbon.
The Knights have won 5 of 13 games to go with a draw so far this year, but are just two points outside of the eight, and only a single point behind the Roosters who have been well short of expectations this year.
Kick-off is set for 3pm (AEST) on Saturday at McDonald Jones Stadium.