In 2020, Hjerpe's freshman year at Oregon State, he made six relief appearances and went 1–1 with a 5.25 ERA before the season was cancelled due to the COVID-19 pandemic. As a redshirt freshman in 2021, he was named the Pac-12 Pitcher of the Week as well as the National Player of the Week by Collegiate Baseball Newspaper on May 24 after he gave up one run and two hits while striking out 11 in a 3–1 win versus the seventh-ranked Arizona Wildcats.[5] For the season, he pitched in 17 games (making 16 starts) and went 3–6 with a 4.21 ERA and 98 strikeouts over 77 innings.[6]
Hjerpe entered the 2022 season as Oregon State's number one starter and garnered numerous preseason All-American honors.[7] On March 22, he was named Pac-12 Pitcher of the Week for the second time in his career after pitching seven innings and giving up one hit while striking out 12 in a 21–0 win versus the Arizona State Sun Devils.[8] On April 1, in a 1–0 loss versus the Stanford Cardinal, Hjerpe threw eight scoreless innings and struck out 17 batters, tied for the most in school history, and was once again named Pac-12 Pitcher of the Week and National Player of the Week by Collegiate Baseball Newspaper.[9][10] At the end of the regular season, he was named to the Pac-12 First Team as well as being named an All-American.[11][12] Hjerpe was scheduled to start the first game of Oregon State's Super Regional versus Auburn University, but was scratched due to an undisclosed illness.[13] He pitched in Game 2 of the Super Regional and passed former Oregon State pitcher Luke Heimlich for the school record for most strikeouts in a season with 161.[14] Hjerpe finished the 2022 season having started 17 games, going 11–2 with a 2.53 ERA, 161 strikeouts, and 23 walks over 103+1⁄3 innings.[15] He was named the 2022 National Pitcher of the Year.[16]
Hjerpe began the 2023 season making his professional debut with the High-APeoria Chiefs.[19][20] He missed time during the season due to an elbow injury that required surgery.[21] Over ten games (eight starts), he went 2–3 with a 3.51 ERA and 51 strikeouts over 41 innings.[22] After the regular season, he played in the Arizona Fall League with the Scottsdale Scorpions.[23]
To begin the 2024 season, Hjerpe started all 11 games he appeared in for Peoria, posting a 0–3 record with a 3.35 ERA and 56 strikeouts over 37+2⁄3 innings. He was promoted to the Double-ASpringfield Cardinals in June, starting in four games with a 2–1 record and 3.07 ERA before being shut down for the season in early July with an elbow injury.[24]
On April 15, 2025, it was announced that Hjerpe would miss the entirety of the season after undergoing Tommy John surgery.[25]
