The Tommies (4-2 overall, 2-1 MIAC) ended an eight-game losing streak to Gustavus (4-3, 2-2) dating back to 2002. The Tommies shot 58% from the field to 34% for Gustavus, which made just 4-of-20 from 3-point range.
UST also outrebounded Gustavus 34-24 to help first-year coach Ruth Sinn get her first road victory.
Junior forward Melissa Gotchnik had 16 points and nine rebounds, and junior guard Melissa Keiser had 13 points, including seven in a game-closing 16-6 run over the final 7:15.
The Toms led by 10 points at halftime and the Gusties never got closer than four in the second half.
Opdahl now has 931 points in 84 career games.
Bri Monahan had 17 points and Jess Vadnais scored 14 for the Gusties.
St. Thomas plays Sunday at Division II St. Cloud State (3 p.m.) then is off until a Jan. 2 MIAC home game vs. Augsburg.
The Gusties host Carleton Saturday night.