BOSTON — The crescendo at Fenway Park was unlike any other this season.

The Red Sox, a floundering club unable to establish consistency, orchestrated a seven-run eighth inning Sunday to overcome a four-run deficit and defeat the Oakland Athletics 7-4 in the teams’ series finale. It was Boston’s biggest win of 2015, as the Red Sox received contributions from up and down the lineup while showing exactly the type of energy and fight they seemingly lacked in their 57 games prior.

“The way in which we won this one probably has a chance or an opportunity to have as big an impact as the win itself,” Red Sox manager John Farrell said. “To put together the number of at-bats we did in that inning. You never anticipate scoring seven runs in an inning, but to see the number of guys who contributed inside that. It was a much-needed win.”

The Red Sox have failed to build momentum all too often this season. They appeared headed down a similar path Sunday, as the offense sleepwalked through the first seven innings despite back-to-back encouraging performances Friday and Saturday. Clay Buchholz, who looked like an ace in his last four starts, couldn’t make it out of the fifth inning before Farrell summoned Steven Wright from the bullpen.

But everything changed with one swing. Rusney Castillo led off the bottom of the eighth with his first home run of 2015. It knocked A’s starter Kendall Graveman from the game and lit a fire under the Red Sox that Oakland simply couldn’t extinguish until it was too late.

Dustin Pedroia followed Castillo’s home run with a hot-shot single to the left side off reliever Evan Scribner and Brock Holt added to the rally with a knock. Hanley Ramirez, fresh off a three-hit performance in Saturday’s middle game, cut Boston’s deficit in half with an RBI single that plated Pedroia and sent Holt hustling from first to third.

“It kept building,” Farrell said. “The solo home run by Rusney jump-starts us, and then Pedey and Brock with back-to-back base hits and you can feel it happening there. Knowing that Hanley’s swinging the bat as well as he has the past seven to 10 days, you’re always a swing away with Hanley at the plate.

“We continued to pass the baton and it was the building of a big inning.”

It didn’t take long to realize that something was brewing in Boston. There was a positive buzz throughout Fenway Park — something that hasn’t always been the case amid the team’s early-season struggles — and the Red Sox seemed like they were playing with confidence, creating a sense that this particular afternoon would be different than all of the other ones in which the team flopped.

David Ortiz pulled the Red Sox to within one run with a sacrifice fly to left field off Drew Pomeranz. Pablo Sandoval, who sat the last two games after hitting rock bottom in Thursday’s loss, then smoked a line drive over left fielder Mark Canha’s head against Tyler Clippard, setting up runners at the corners.

Mookie Betts pinch-ran for Sandoval and swiped second base. It paved the way for Xander Bogaerts to propel the Red Sox in front with a two-run, two-out, two-strike double off the Green Monster.

Bogaerts, who has been a bright spot for Boston this season, especially of late, pointed toward the Red Sox’s dugout as his teammates went bananas with enthusiasm. The comeback was complete.

“I remember just looking at the scoreboard at the beginning of the bottom of the eighth, 4-0, just trying to think how we can get some runs,” Bogaerts said. “That was probably the biggest win for us this year, for sure.”

Alejandro De Aza, who pinch-hit for Sandy Leon, and Castillo, who started the noise, added some insurance with back-to-back RBI singles that only increased the volume at Fenway Park. It was a relentless assault that was reminiscent of 2013, when dramatic wins became the norm for a lively bunch.

The Red Sox have had a few good wins this season. And in many instances, they’ve followed them up with duds, meaning one shouldn’t assume they’ll touch down in Baltimore and run roughshod through the last-place Orioles. But there was something different about Sunday’s victory, or so it seemed.

The Red Sox, who entered the game with a 1-26 record when trailing after seven innings, finally showed an ability to scratch, claw and battle for a victory.

The atmosphere at Fenway was raucous. And the effort was music to Boston’s ears.

Thumbnail photo via Twitter/@RedSox