But as the season slips away, the Knights coach has taken aim at referee Gerard Sutton over a handful of key decisions in Sunday's game at Shark Park.
After Newcastle's Phoenix Crossland missed a field goal at the other end, Cronulla raced deep into Newcastle's half through right winger Sione Katoa.
There, back-up five-eighth Atkinson slotted the match-winner from 20 metres out with 30 seconds remaining in the first period of extra time.
"That's going to make it hard now," Knights coach O'Brien said of making the finals.
"We probably need to rely on some results now that we've dropped that one."
Atkinson slots his match-winning field goal. (Mark Evans/AAP PHOTOS)
The win keeps an error-prone Cronulla two points inside the top four and means the Knights are now unable to end the season with a winning record - usually the benchmark for a top-eight finish.
Newcastle could still sneak into the post-season, but it would require a multitude of results falling their way, and likely also a significant improvement in their inferior for-and-against (-90).
Making this more difficult, Tyson Gamble suffered a broken hand and left the field with six minutes to play, though O'Brien would not put a timeline on the five-eighth's recovery.
"He'll be out next week, anyway," he said.
A frustrated O'Brien criticised referee Sutton's decision to deny both the Knights and Sharks potentially game-winning field goals towards the end of regulation time on the grounds of illegal blockers.
It comes after the NRL flagged in the pre-season it would review each field goal this year to determine whether the attacking team's non-kicking players were in the line impeding the defence.
"I thought theirs was a field goal as well," O'Brien said.
"Graham (Annesley, NRL head of football) will find a way to dress it up."
Ronaldo Mulitalo was busy for the Sharks, scoring a late try and dealing with Mat Croker. (Mark Evans/AAP PHOTOS)
O'Brien also criticised the decision to sin-bin Crossland for holding Jesse Ramien down while the Sharks attacked the Knights' line in the shadows of half-time.
Crossland had given up a six-again a minute earlier and appeared to have been preventing Ramien from playing the ball as the Sharks attacked the line in the final seconds of the half.
"I'd be shocked if anyone agreed," O'Brien said of the Crossland sin-binning decision.
"That was the third infringement for the half. I think we had one for offside from a scrum, which happens a lot, and we only had one for slowing the ruck down. On the third one he puts a bloke in the bin.
"This team is fighting to stay in the competition and then you get that done to you?Â
"Usually someone gets warned that it's going to go there. He went there on the third penalty. Fair dinkum. It's ridiculous."
Without the injured Braydon Trindall and Nicho Hynes, the Sharks struggled for attacking polish after rushing to a 10-0 lead against a violent Cronulla breeze. In total, the home side made 17 errors.
Atkinson ended with a try, two conversions and a penalty, and his heroic 84th minute field goal. (Mark Evans/AAP PHOTOS)
Playing for their season, Newcastle threatened to clear the biggest hurdle on the road to a fairytale finals berth when Dane Gagai crossed to give the 12-man Knights an 18-10 lead.
But Cronulla finally overcame their knack for dropping the ball in attacking territory when left winger Ronaldo Mulitalo crossed on an overlap in his first game back from a knee injury.
With seven minutes to play, Atkinson nailed the sideline conversion that sent the game into golden point.
He saved his most important strike for later, though.
"There was a lot of stuff there that wasn't pretty but a lot of good stuff as well," Sharks coach Craig Fitzgibbon said of the win.
"The ability to hang in. We had so many players out of position and still found a way to get it done. That's important at this time of year."
10°C