OAKLAND -- About halfway toward first base, after elevating a full-count fastball from right-hander Lance Lynn on Tuesday night, Kyle McCann looked up at the right-field Coliseum scoreboard and began doing math in his head.

Based on the exit velocity reading of 106.3 mph, McCann knew he struck it well enough. But given the chilly East Bay weather and the height of the fly ball, he was unsure whether it would actually leave the ballpark.

“It was a bit of a moonshot,” McCann said after the A's dropped a 3-2 decision against the Cardinals. “I was praying it was going to go out of the yard. I know in Oakland, depending on the night, that it could get thrown back into fair play.”

Jordan Walker chased after the ball until he met the wall in right and watched it bounce off the first row of seats, to which McCann yelled in elation and smacked his chest while rounding first. The solo shot in the fifth inning was his first Major League home run.

“To see it go out was amazing,” McCann said. “Dream come true. … I was fired up running around the bases. It was a great moment.”

Sitting in a glass case atop McCann’s locker in the A’s clubhouse postgame was his first home run ball.

“I’m going to kiss [the ball] later,” McCann said. “I’m going to keep it, for sure. I’ll have it displayed somewhere in my house.”

While the rookie’s blast put the A’s ahead by a run in the fifth inning, Oakland’s bullpen was unable to hold the lead. Still, the final result does not diminish the feel-good story of McCann’s perseverance.

While playing college ball at Georgia Tech, it was McCann’s propensity to hit “moonshots” like the one on Tuesday that helped raise his stock heading into the 2019 MLB Draft, which saw the A’s select him as a fourth-round pick. In the following years, McCann was a fixture on Oakland’s Top 30 prospects lists.

His 55-grade power was a standout tool as he moved through the club’s farm system with solid offensive numbers at each level. But while McCann could hit the ball hard, it was his lack of polish behind the plate that gave the A’s pause as to whether he was ready for the big leagues.

After grinding through 355 Minor League games and a couple of heartbreaking cuts from the roster during the final week of Spring Training in 2022 and ’23, McCann got his first call to the Majors as part of Oakland’s Opening Day roster this season.

“Kyle has come a long way,” A’s manager Mark Kotsay said. “He came into camp in great shape and motivated to make the roster, and he performed. He did everything he needed to do to be on this roster, and really made an impression and built trust, which is as key component to being a catcher.”

It’s not an easy role for the 26-year-old catcher. As the backup to Shea Langeliers, McCann is mostly confined to the bench, save for one or two days out of the week.

So far, the A’s have been pleased with how he’s handling the situation. After catching JP Sears last week in a start that saw the left-hander carry a no-hitter into the seventh inning at Globe Life Field against the Rangers, the two connected again on Tuesday for another solid outing. Sears limited St. Louis to one run on two hits and three walks in five innings.

“That’s a tough role as the backup catcher,” Sears said. “It’s hard to get at-bats and hard to feel comfortable up there. But he looks comfortable. That’s a big part of it. He’s just trying to be who he is and not do more than he has to do. He’s got such a good swing and a good plate presence, and he calls a great game.”