MESA, Ariz. -- It’s rare that a simple Spring Training game can leave a pitcher a nervous wreck. For A’s right-hander Adam Oller, though, there was nothing simple about this one.

Over the first six years of his professional baseball career, Oller had never attended a Major League Spring Training, which had actually become a running joke with him and his agent as one of the last things to cross off the “to-do” list in his career. That all changed earlier this month, when Oller was added to Oakland’s 40-man roster as one of two pitchers acquired from the Mets in exchange for Chris Bassitt.

Upon learning earlier this week that he would take the mound to start Wednesday's game against the Cubs -- a 5-4 loss for Oakland -- Oller immediately battled to suppress the anxiety. He woke up and stood in front of a mirror for a pep talk with himself. Once the team bus pulled up to the ballpark, Oller checked his heart rate: It was 95, which is well above average for a resting heart.

“I was like, ‘This isn’t good,’" Oller said. “Once I faced the first hitter, it cooled off for me a little bit. I got the first out and I was like, ‘OK. We’re good. Everything is fine. Just go.’ But the nerves for warming up and everything like that, that’s hands down the most people I’ve ever thrown in front of.”

The crowd of 8,779 fans at Sloan Park might as well have been 40,000 fans at Wrigley Field with the emotions Oller was going through. Among those in attendance were his father, Mike, who flew in from Knoxville, Tenn., on Tuesday night. Also on hand was D.J. Carrasco, a former big league pitcher and a mentor for Oller.

Responding to the adrenaline overload, Oller fired off five straight fastballs of 94 mph to Cubs leadoff hitter Nick Madrigal before retiring him on a groundout. Collecting himself shortly after, the 27-year-old righty turned in an impressive spring debut, striking out four of the eight batters he faced while allowing one run on two hits and one walk across two innings.

“He was excited to be out there for the first time,” said A’s catcher Sean Murphy, who had two RBIs and was credited by Oller for helping him navigate through the early jitters. “I thought all his stuff was looking good. His fastball was playing really well. Breaking ball was a little erratic. I’m sure he’ll tell you that he wasn’t as sharp as he could be, and even saying that, I thought he looked pretty good.”

Oller’s long professional baseball journey began as a 20th-round pick by the Pirates in the 2016 MLB Draft. After fighting to keep his playing days alive with stints in independent ball and the Australian Baseball League from 2019-20, he landed with the Mets as a selection in the Minor League phase of the 2020 Rule 5 Draft.

It was in New York’s system that his fortunes turned. Named the Mets’ organizational Pitcher of the Year for a 2021 campaign in which he went 9-4 with a 3.45 ERA and 138 strikeouts over 120 combined innings at Double-A and Triple-A, Oller improved his stock to the point where he arrived as the A's No. 25 prospect, per MLB Pipeline.

So what changed?

“Honestly, the biggest part of it was the mental aspect of it,” Oller said. “It wasn’t a matter of stuff, I just was putting myself in bad situations. We worked on throwing stuff over the plate and seeing what happens. Sure enough, I do that and start getting ahead in counts a lot better.

“Between 2019 and now, gaining some velocity and adding some more stuff to my repertoire -- gaining a cutter and better changeup -- ended up helping me out to where last year it wasn’t just fastball-breaking ball. That made a huge difference.”

Oller's stuff sure played well in his first start. All four of his strikeouts were of the swinging variety. Between the fastball that sat 94-95 mph and the improved changeup, Oller produced 10 swing-and-misses.

The A’s have at least one rotation spot up for grabs, with James Kaprielian (right AC joint irritation) expected to miss the start of the regular season. More slots could open up depending on the future of Sean Manaea and Frankie Montas, whose names remain heavily discussed in trade rumors.

Between that and reports of MLB potentially expanding rosters to 28 players for the first month of the regular season, Oller could have more than a decent shot at making the Opening Day roster. But until the day comes when he can take the field in an official Major League game, Wednesday’s experience will remain the most special day of his baseball life.

“I feel like I’ve thrown in a lot of big games before, but this one takes the cake,” Oller said. “Dad was here in town watching. Plus I had all my friends watching on TV and texting me afterward. A really cool experience.”