JUPITER, Fla. -- With All-Star catcher Willson Contreras digging into the box, manager Oliver Marmol peering through a screen behind home plate and president of baseball operations John Mozeliak nearby and watching intently, Cardinals reliever Riley O'Brien couldn’t help but briefly think on Saturday about how far his career has come and how it almost never was if not for him making the most of a chance opportunity years earlier while pitching in college.

A starter-turned-reliever during a relatively unspectacular two-year stint at NAIA’s College of Idaho, O’Brien got word on the morning of April 23, 2017, that he needed to start that day against conference powerhouse Lewis-Clark State. With dozens of MLB scouts in the stands, O’Brien consistently threw pitches at 95 mph for the first time in his career -- three miles per hour faster than he had ever hit before. Famed Rays scout Paul Kirsch was one of the talent evaluators on hand that day -- albeit to watch players on the other team -- but he came away so impressed with O’Brien that he convinced the Rays to pick the right-hander in the eighth round of the 2017 MLB Draft.

“I went [to junior college] the first couple of years. Things didn’t go too well. I was in my head with my mechanics and at that point, I thought baseball might have potentially run its course,” O’Brien remembers. “I didn’t really have any [college] offers and my JUCO coach had played at College of Idaho and gave me a reference. I figured some things out there and then in my senior year, everything clicked.

“Against Lewis and Clark State, they told me right before we left the hotel I was starting and there were tons of scouts there watching, and I had a pretty good game,” added O’Brien, who struck out four and allowed just two runs over five innings. “I’d say that game was really a turning point for me in my career.”

Flash forward seven years to Saturday, when O’Brien was on the mound and throwing his first live session against Cardinals hitters, and he rallied back from a 3-0 count to strike out Contreras on a high fastball. Later, he got lefty slugger Nolan Gorman also to whiff. It was a pinch-me type of moment for O’Brien, who has mostly toiled in the Minor Leagues for the past six seasons. The 29-year-old O’Brien, who was acquired by the Cardinals from the Mariners in a November trade, is hopeful that his stellar pitching this weekend is a harbinger of things to come in 2024.

“Obviously, if I’m going to be in the big leagues, I’m going to be facing the best hitters in the game, so to get to face some of those [Cards starters], it was cool,” said O’Brien, who has twice before made it to the big leagues with the Reds and Mariners. “I want to face the best hitters out there, because that’s what I’ll be seeing in the big leagues.”

Marmol came away impressed with what he saw from the newcomer, darting his hand in two different directions to signify the cut on O’Brien’s fastball and the break on his slider. Contreras, who has caught and hit off O’Brien, is eager to see more from the right-hander.

“He looked really good,” Contreras said. “Now, we’ve got to see him do it in games.”

O’Brien has always been something of a late bloomer, so it’s probably no surprise that it’s taken him a few years to find traction in pro ball. He grew from 5-foot-3 to 6-foot-3 from his freshman year of high school to his junior baseball season. He was so tiny early on that former Red Sox second baseman Dustin Pedroia -- all 5-foot-9 of him -- was his favorite MLB player, even though he lived in Seattle and loved the Mariners.

Only two MLB bullpens had fewer strikeouts than that of the Cardinals in 2023, so the organization dumped much of its pitch-to-contact theories and sought more swing-and-miss stuff this offseason. That led them to O’Brien, who fanned 86 batters in 55 innings last season with Triple-A Tacoma. He’ll be trying to crack a big-league bullpen that includes incumbents Ryan Helsley, Giovanny Gallegos, JoJo Romero, Andre Pallante and Matthew Liberatore and newcomers Andrew Kittredge, Keynan Middleton, Nick Robertson and Ryan Fernandez.

“I’m focusing on attacking guys more, and I treated every bullpen this offseason like it’s an 0-0 count and I’m trying to get ahead,” said O’Brien, who was bothered by walks in his two MLB outings. “I want to throw my [fastball] in the heart of the plate and work from there. Then, I can use my slider. When I’m dialed in, I can throw it for a strike and expand off it. That’s what I’m trying to show.”