With one fluid swing, Ryan Mountcastle showed why the Orioles missed him.

Mountcastle, who has been out since May 31 with a right hamstring strain, hit a home run in his first at-bat since being activated off the injured list earlier Friday. But he did so as the designated hitter, not the first baseman. And that’ll likely be his home for most of the remainder of the season as he balances playing time with Coby Mayo.

Mountcastle’s home run, plus Adley Rutschman’s two-run shot and Tomoyuki Sugano’s stellar seven-inning start, led the Orioles to a 3-2 win over the Athletics to open a three-game series at Camden Yards.

“It was good to get off to a nice early start, and Tomo pushed the way he did and our bullpen to come in and shut the door was a lot of fun,” Mountcastle said. “Just happy to be back with those guys.”

Much has changed since the last time Mountcastle was in the lineup. The Orioles had just fired manager Brandon Hyde and were adjusting to life under interim manager Tony Mansolino. The clubhouse, while struggling with injuries, largely resembled the one they had on opening day.

Over two months later, the same can’t be said. The Orioles sold off nine players at last week’s MLB trade deadline. Had Mountcastle been healthy, he might have joined them. He has one year left before becoming a free agent, but the Orioles, by trading Andrew Kittredge, Ramón Urías and Ramón Laureano, showed they weren’t afraid to send away players with another year of team control left.

Now the Orioles must grapple with how to handle Mayo, the budding rookie they believe is their future at first base. Mayo’s playing time increased when Mountcastle went on the injured list, and he’s been in the lineup daily since the team traded Ryan O’Hearn. While he went 0-for-2 Friday, he has shown notable improvement with daily playing time, including making a diving catch in foul territory against the Athletics.

Mansolino said prior to Friday’s game that he needs to finalize the plan with Mountcastle, but it seems as if this lineup was a good indication of what to expect. Mountcastle, a two-time Gold Glove finalist who spent his early career a defensive nomad before finding first base, should primarily be the designated hitter to allow Mayo the time to develop at first base.

The rest of the season is about seeing which parts of the roster move forward with the Orioles in 2026, and giving Mayo the time to prove he can handle being a daily major league first baseman is a big part of that. Although he has just four home runs, the team believes he has more power to offer.

“I mean he’s not super new to the position but you know still learning, so whatever they want me to do, I’m willing to do,” Mountcastle said. “And, yeah, just getting him some more reps over there, I think it’ll be good for him.”

At his best in 2021, Mountcastle hit 33 home runs. Since then, his production has dipped — he hit only 13 last year and had just two in 52 games prior to Friday. Mountcastle said he felt comfortable during his rehab assignment with Triple-A Norfolk — where he hit three home runs in nine games — and that he was seeing the ball well, which he thinks translated to the majors.

“He’s a really good hitter,” Mansolino said. “Go back a few years ago and he hit 30-something homers in his rookie year or so. He has not done that probably at the rate he’d like here the last couple years, but I do know that there are some swing adjustments while he was on the IL, and I have a really good feeling about him these last couple months.”

Sugano, too, is fighting for his future. He’s on a one-year, $13 million deal with the Orioles, and if he plans to play another year in Major League Baseball he’ll need to prove he can put together more nights like this. He pitched seven innings for the first time since June 3, allowing one run on five hits.

“The split was hard and sharp,” Mansolino said. “A lot of confidence out there right now. This is kind of what he looked like early in the year when he came in there. He just had probably four or five bad starts. Probably adjusting to the schedule, the workload of pitching in the USA and in this league. He’s been pretty good here for a while. The split was hard; it was sharp. The fastball had some life to it. That’s a pretty good lineup right there that he rolled through.”

The Orioles’ new post-deadline shutdown duo of Yennier Cano and Keegan Akin pitched the eighth and ninth, Cano allowing one run but keeping the team in position to win.

This article has been updated.