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]
To open the 2023 season, Hjerpe was assigned to the
Peoria Chiefs of the
High-AMidwest League with whom he made his professional debut.[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] He was selected to play in the
Arizona Fall League with the
Scottsdale Scorpions.[23]