It was back in 2009 and, although the county already had Leinster in a headlock, they were still at a stage of their development where there were bigger and badder teams out there who thought little of roughing them up, and shaking them down for pocket money.
They managed only two wins and a draw through the spring. It was just about enough to keep them out of the clutches of Division Two but the year ended with a 17-point mauling at the hands of Kerry in an All-Ireland quarter-final.
“Five All-Irelands was a very distant thought at that stage,” said O’Sullivan.
Six of those who featured that day were still on the panel last Sunday when Mayo were bettered in a madcap, no-holds barred scrap. They have played central roles in the transformation of a team that just couldn’t win when it mattered most to one that doesn’t know how to lose.
O’Sullivan wouldn’t go quite that far.
“No, we were beaten in the league final by Kerry this year. Last year that (Mayo) game went to a replay and we won by a point, won by a point (again this year). There are very fine margins there. Cillian O’Connor had a kick late in the game and it hit the post.
“If that had gone over, it would have been a completely different story. Very fine margins. We know that there isn’t much between us and the other teams.
“The resilience that this team has built up over the years is the difference coming through those hard battles.
“The experience of that really shows in those last couple of minutes. We just controlled the ball very well. It was probably not the prettiest thing to watch but the key thing was to hold on, maintain possession, and see the game out in the last few minutes. Thankfully we were able to do that.”
The margins are, frankly, ridiculous. Maddening if you are a Mayo fan. The biggest wins in the past 10 All-Ireland finals have been by four points. Five have been settled by a single point. All by Dublin.
O’Sullivan apportions much of the credit for that to Gavin, his military background and the blanket of calm and composure he brings.
It was apparent in the 76th minute on Sunday when Dean Rock blocked out the hum of 82,000 people and a flying GPS system to nail the decisive kick from a free. One year’s work distilled into one man’s boot.
“He has nerves of steel,” said O’Sullivan.
“I owe him a debt of gratitude now for the next however many years. He really stepped up to the plate there, pressure on, a pressure kick and he just executed it absolutely perfectly. He’s been doing that all year.
“It’s not an easy thing to do. He is some talent. We’ve so many guys on the team like that, that have stepped up to the plate.
“Diarmuid Connolly came on, Cormac Costello came on. These guys came on and really added something.”