Pennsville advanced to face No. 1 Bishop Eustace in the semifinals at 1 p.m. Sunday at Washington Township. The title game is set for 5 p.m. Sunday at Eastern.
"We're in it to win it," O'Brien said. "It's huge to these kids."
Junior Ryan Shinn hit a solo home run in the sixth, and senior lefty Zach Gakeler pitched five-plus innings for No. 3 Northern Burlington (19-5).
"We know we don't play the same competition of the bigger teams," said Walker, referring to Pennsville's status as a Group 1 team that plays in the Tri-County Conference's Classic Division. "But we feel like we can compete with them when we get a chance."
O'Brien said he would use his top pitchers in an effort to capture the Diamond Classic crown even though the Eagles open the South Jersey Group 1 tournament on Monday.
"I told the kids I might use Walker, [Jared] Cornman, [John] Straub, and [Colin] Rieger," this weekend, O'Brien said. "Three kids raised their hands and said they would take the ball Monday."
Walker, who improved his record to 8-0, threw 100 pitches. He mixed a sharp fastball with a tight curve and stayed down in the strike zone.
He was at his best in tight spots, working out of trouble in the first, third, and fourth and inducing a ground-ball double play in the seventh.
"Mitch was awesome," O'Brien said.
Said Walker: "I strive to be a bulldog out there."
Shinn's home run tied the game at 1 in the top of the sixth after Pennsville senior Josh Shimp gave his team a 1-0 lead with an RBI single in the bottom of the fifth.
Pennsville broke the game open with seven runs in the sixth. Rieger started the rally with a single, and Austin Moore reached on a bunt single. Chase Flickenger followed with another bunt that Northern Burlington misplayed, allowing one run to score.
"It's strange how bunts are really hard to defend," O'Brien said.
After a hit batter and Peyton Sorells' bases-loaded walk, Lankeneau poked a two-run single to right, and Walker lifted a towering fly to left that carried over the fence to make it 8-1.
"I thought it was a sac fly," Walker said of his second home run in two Diamond Classic games this season. "I was like, 'All right, it got one run in.' But it kept carrying."
Northern Burlington 000 001 0 - 1 7 2
Pennsville 000 017 x - 8 11 1
WP: Mitch Walker. LP: Zach Gakeler.
2B: NBC-Adam Rapp 2, Tyler O'Dell. P-Dylan Cummings.
HR: NBC-Ryan Shinn. P-Mitch Walker.