Boston Bruins defenseman Rob O'Gara (44) on the ice during...

Boston Bruins defenseman Rob O'Gara (44) on the ice during the pre-skate before a game against the Anaheim Ducks, on November 15, 2017, played at the Honda Center in Anaheim, CA. Credit: AP / John Cordes

GREENBURGH, N.Y. — Rob O’Gara brought an AHL reputation as a dependable defensive defenseman out on the ice for his first practice in a blue Rangers jersey Wednesday. But his new coach found fault with one thing — the rookie’s old pledge of allegiance.

“He’s got one major flaw: I guess he was a big Islander fan growing up,” Alain Vigneault said, smiling. “We’re going to have to work on that. I talked to him about that this morning.”

The 24-year-old former Yale star was born in Islanders country and attended games at Nassau Coliseum. His dad, Brian, was a die-hard fan who worked across the street. Still does, O’Gara said.

But the rebuilding Rangers acquired the 6-4, 215-pound O’Gara and a 2018 third-rounder from Boston Tuesday in exchange for veteran defenseman Nick Holden. O’Gara will make his debut Thursday night in Montreal.

“It was a little bit of a shock at first,” O’Gara said, “but then a lot of excitement, talking to family, friends.”

He came with several Long Island ties. O’Gara was born in Massapequa and spent his first few years there, but he grew up primarily in Nesconset. He attended Smithtown East High School for three years and was with the Long Island Royals’ travel program out of Kings Park from ages 13-16 after falling in love with hockey while watching the Islanders in Uniondale.

“But right now I’m a Ranger fan,” O’Gara said.

The Bruins drafted him in the fifth round in 2011. O’Gara played mostly with Providence in the AHL after turning pro late in the 2015-16 season. He did play three games with Boston last season and eight games early this season. But the organization was stocked defensively. So O’Gara is happy to get this opportunity.

“It’s no secret that there is one,” O’Gara said. “There’s time here to prove myself. It’s a clean slate.”

The lefty will start out being paired with John Gilmour.

“[O’Gara is] a young player trying to find his way in the NHL,” Vigneault said. “We’re going to give him an opportunity here to show what he can do.”

O’Gara remembers watching his Islanders favorites, Kenny Jonsson and Brad Isbister, and the Coliseum just being “a special place as a kid.”

So he said putting on the Rangers’ practice jersey was “a little bit [surreal], but it’s a job. It’s an honor to wear this, any sweater in the NHL, but the story behind this is incredible, even though I wasn’t a fan of it as a kid. It’s really special.”

Notes & quotes: Defenseman Marc Staal will return after missing eight games because of a cervical strain . . . Defenseman Ryan Sproul was assigned to Hartford . . . Goalie Alexandar Georgiev will start and make his NHL debut in Montreal . . . Left wing Chris Kreider (rib resection) will miss his 24th game, but Vigneault said he could return Friday or Sunday . . . Vigneault said defenseman Ryan McDonagh (upper body) is “a matter of days” from returning to practice. But he’s being mentioned in trade speculation.

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