Immediately after the incident, Elliot recorded a four-minute social media video in which he admitted knocking O'Brien unconscious.
"What's up with these middle-aged f****g white boys ... who think they can take on someone like me?" He said in the video.
During the three-day jury trial in the Napier District Court, O'Brien admitted that he threw the first punch in the dispute, after he had been backed up against a display cabinet and Elliot had pushed his wife.
Elliot told police he was acting in self-defence after O'Brien "got me a good one".
In his summing up, Judge Collins noted that the Crown had called the two protagonists a "hopeless mismatch".
The court was told that Elliot, 45, was 140kg and had been training in martial arts since the age of 10. O'Brien, now 60, said he was 175cm tall and 80kg.
The judge told the jury it was up to the Crown to prove that Elliot did not use reasonable force in defence of himself.
But if the jury were sure that Elliot struck O'Brien at least once after his first punch and while O'Brien was on the ground, then "self defence is not available".
Judge Collins said that every Crown eyewitness, apart from O'Brien himself, who had been unconscious, had said they saw Elliot punch O'Brien while he was on the ground.
The jury took one hour and 50 minutes to reach their two guilty verdicts.
Elliott was remanded in custody for sentencing on November 2.
The judge said Elliot had a domineering and manipulative personality and liked to dominate any environment he was in.
Imprisonment was likely and "I am concerned about the risks you present" if given bail, the judge said.
Stephen Elliot is the brother of former All Black, and Hurricanes and Chiefs rugby player Hika Elliot, who is now playing in France.
