Hammered in their previous two outings at the Sydney Cricket Ground, the Knights produced the upset of the weekend on Saturday and even O'Brien admitted they could have won by more.

Huge outsiders to win and tipped to go backwards this season, they ran in four tries and defended strongly to claim victory with a quality collective performance.

O'Brien wasn't getting carried away but he credited new halves pairing Adam Clune and Jake Clifford, who had not played a single NRL match together before, as key pillars of the victory.

"Really good temperament from both of them," O'Brien said. "They allow [Kalyn Ponga] to do what he does.

"The control of the game from Adam in particular was really good. He is a smart guy on and off the field. It allows Cliff to be Cliff too, he runs the ball well. There's stuff there that we want to get better at ... but they're doing a pretty good job. They've had a good summer the halves."

First-time captain Kalyn Ponga said he and the halves had "just been trying to work together and share the load".

"Know our roles and nail our roles," he said. "I think that was quite prominent in our performance. There was a patch in that first half where we were a bit sideways, but ... it's being on the same page and working together. Across the board, everyone had their part in today's win."

They Knights looked likely but against one of the competition's top sides it was never a certain result until late on. Their chances greatly improved in the 57th minute when, leading 10-2, five-eighth Jake Clifford scooped up a dropped ball and sprinted more than 50 metres to score between the goal posts.

A few minutes later the Knights were worryingly reduced to 12 men when Phoenix Crossland was sin-binned for a professional foul, but they desperately defended their own try line to keep the Roosters at bay.

"The 10 minutes we're down to 12 [players], they didn't break us," O'Brien said. "We showed a tonne of resilience defensively."

Knights tyro Dom Young exemplified the defensive effort, twice forcing his opposing winger Daniel Tupou over the sideline while Crossland was off the field.

The 20-year-old Englishman was rewarded with a try of his own in the 73rd minute, impressively diving over the try line and putting the ball down with one hand.

In his first game back in the red and blue Dane Gagai scored the opening try, running in untouched after gathering a ball lost by James Tedesco 20 metres out.

The Knights' other centre Bradman Best showed what weapons he and Gagai could be this season, putting on a classy fend and flick pass to allow Enari Tuala to score the side's second try eight minutes before halftime.

There were other opportunities the Knights didn't make the most of, but with an improved attacking showing and unlikely first-up win they will soon be forgotten.

"I thought we left a couple of tries out there as well with some execution, which you're going to get in round one," O'Brien said.

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