OAKLAND -- Brent Rooker had Monday circled on his calendar for weeks. It was the date that his favorite country singer, Zach Bryan, would be in town performing at the Oakland Arena, which shares the same parking lot as the Coliseum.

There was just one problem: The A’s were also opening up a three-game series against the Royals that same night.

With tickets for him and several of his teammates in hand, Rooker knew they would be cutting it close to Bryan's stage time next door. Things got even more dicey as extra innings loomed in a tie game entering the bottom of the ninth inning. At that point, Rooker decided to take matters into his own hands.

Stepping to the plate against Royals reliever Dylan Coleman with Ryan Noda on first base and no outs, Rooker took two pitches for balls before pouncing on a hanging slider and sending it over the high wall in left field for his 21st home run of the season, lifting Oakland to a 6-4 walk-off victory.

“I know he really wanted to get to this concert next door because he knows the manager,” manager Mark Kotsay said of Rooker. “Rook has been swinging the bat pretty well as of late. It was a good feeling to win tonight. We coughed up a 4-2 lead, and I would have hated to be on the other end of that game tonight.”

After making it next door to the arena, Rooker wound up on stage with Bryan and his band -- wrapping a pretty great night for the A's 2023 All-Star slugger.

While Rooker’s blast came on a night in which the A’s combined for four homers totaling 1,647 feet, according to Statcast, the one that garnered the most attention came off the bat of rookie Zack Gelof -- Oakland’s No. 3 prospect and the No. 94 prospect in baseball per MLB Pipeline -- who continued what has been a historic start to his big league career.

Gelof’s solo shot to lead off the bottom of the third was his ninth home run, making him the fastest player to reach six homers (32 games) in A’s franchise history.

“He continues to set records here, open eyes and make a statement that he belongs,” Kotsay said. “Not only that he belongs, but that he wants to be here for a long time and lead these guys. The quiet leader that he’s becoming, it’s something that I find joy in watching daily.”

Due to Gelof’s eye-popping statistics -- he has now reached base in 18 straight games and his 1.175 OPS for the month of August entering Monday ranked third in MLB behind Mookie Betts and Corey Seager -- an underrated aspect of his emergence has been the leadership qualities Kotsay referred to.

Prior to Monday’s game, fellow A’s rookie Lawrence Butler described the impact Gelof has made inside the clubhouse.

“Zack’s one of the leaders now,” Butler said during a segment on A’s Cast. “He comes in and does it the right way. He gets his work done every day. Always in the weight room trying to get better. When I come in here, I just kind of follow him and see what he does. Just try to mold myself to be a player like him.”

Of course, it is hard not to fawn over Gelof’s prodigious hitting abilities that continue to shine. On Monday, he ambushed a first-pitch slider from Alec Marsh and drove it the other way, a projected 428 feet to right-center.

“He has some of the most backside power I think I’ve ever seen,” Rooker said of Gelof. “Every time he does something in front of me, I’m walking on the on-deck circle looking at [the hitting coaches] and go, ‘Man, that kid is really good.’ He just continues to impress.”

With Gelof sustaining his massive success for over a month now, it has reached the point where his teammates and coaches are struggling to conjure up different ways to explain what they are witnessing.

“I don’t even really know how to put it into words,” said A’s starting pitcher Paul Blackburn, who limited Kansas City to two runs on four hits and two walks with six strikeouts over six innings. “The guy doesn’t give at-bats away. Every at-bat, it feels like he’s going to win. He’s a bulldog in the box. It’s just fun to watch him learn and change his approach from at-bat to at-bat against good pitchers.”