After high school, Woo enrolled at Cal Poly San Luis Obispo to play college baseball for the Cal Poly Mustangs. As a freshman in 2019, he pitched to an 8.75 ERA over 23+2⁄3 innings. He returned to play for the Oilers that summer.[4] He pitched only 17+2⁄3 innings in 2020 before the college baseball season was cancelled due to the COVID-19 pandemic. In 2021, he compiled a 6.11 ERA over 28 innings before undergoing Tommy John surgery and missing the rest of the season.[5] Woo started only six of the 31 games he pitched in college, finishing his college tenure with a 4–7 record, one save, 6.49 ERA, and 89 strikeouts in 69+1⁄3 innings.[6]
Woo started 2023 with the Double-AArkansas Travelers.[15] In nine starts, Woo had a 3–2 record and 2.05 ERA with 59 strikeouts in 44 innings pitched.[13] On June 2, 2023, the Mariners announced that Woo would be promoted to the major leagues to start the following day against the Texas Rangers.[16] Starting in place of Marco Gonzales,[17] Woo had one of his worst starts of the season, allowing six runs in two innings.[18][19] He won his first MLB game in his fourth start, besting the New York Yankees on June 22. He finished his rookie season with a 4–5 record and 4.21 ERA, striking out 92 batters in 87+2⁄3 innings.[20]
Despite dealing with several injuries, Woo improved on the mound in 2024. He began the season on the 10-day injured list with elbow inflammation.[21][22] He made his first start on May 10.[23] In his first six starts, he posted a 1.07 ERA, the lowest in Mariners history to begin a season.[24][25] He was scratched from his next start on June 11[26] and made two more starts, earning losses in both outings,[23] before returning to the injured list on June 25 with a hamstring injury.[27] He was activated on July 12,[28] resuming his turn in the Mariners rotation. He finished the 2024 season with a 9–3 record and 2.89 ERA, striking out 101 batters in 121+1⁄3 innings over 21 starts, all improvements from his rookie season.[29] He was the best MLB starting pitcher at avoiding base on balls in 2024, walking 2.6 percent of batters faced.[30]
