As Will O’Brien watched the third and final game of the Nassau Class A baseball championship unfold Wednesday, he wished he was on the mound or at least on the field instead of the dugout.

O’Brien was in Wantagh’s lineup, but a sore arm relegated him to designated hitter out of the No. 3 slot.

“I obviously wanted to be out there helping my team,” he said, “but I knew I can help them by swinging the bat well this series and that’s what I did.”

With one out in the bottom of the ninth inning of a tie game, O’Brien stepped to the plate with the bases loaded.

He worked the count to 2-and-2 and stroked a fastball through the right side, scoring Charlie Interrante and leading No. 7 Wantagh to a 3-2 win over No. 11 Bellmore JFK. The Warriors, who last won the county title in 2010, will face Mt. Sinai Saturday for a spot in the state semifinals.

“It’s a feeling I’ll remember for the rest of my life,” said O’Brien, who went 2-for-3 with two walks. “All I wanted to do was get the ball on the bat and score the winning run. That’s what happened.”

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With O’Brien unavailable and Game 1 starter Jimmy Joyce and Game 2 starter Bobby Hegarty limited to light workloads, Wantagh coach Keith Sachs called on Brendan Haas to take the mound.

Haas, a junior who had started one game all season, turned in six strong innings. He allowed one run — on Mike Schwartz’s first-inning RBI single — and six hits.

Haas said he did not know he would start until Wednesday morning, when Sachs delivered the news at school.

“At that point, I was just trying to stay calm, not get too excited and come out here and do my job,” said Haas, who struck out four and walked one.

Wantagh (22-5) erased Bellmore JFK’s 1-0 lead in the third, when Joyce’s sacrifice fly knocked in Anthony D’Onofrio. But with two outs in the seventh, Joyce allowed a run-scoring triple by Frank Bruno (3-for-4).

Joyce got the run back with a groundout that plated pinch-runner Steve Forman in the seventh.

The teams traded zeroes until O’Brien came to bat in the ninth.

“It was great seeing him get the hit to win the game, especially after he went down with that injury,” Haas said. “It was a great moment for him and the team.”

For Bellmore JFK (18-10-1), the moment was not as sweet. But the Cougars’ season still ends a historic one. The school’s baseball program had never reached the county championship.

“It was exciting. It was great,” said Eric Passman, in his 17th year at the helm. “Both teams played tremendous. You’ve got to give credit to them.”