Thomas, who finished with 20 points, 11 rebounds and eight assists, also made two free throws in the closing seconds as the Ducks (3-16, 1-6 Pac-12) ended an eight-game losing streak and won for the first time in nine games away from home this season.
Amanda Delgado led Oregon with 21 points, all scored from behind the 3-point line where she was 7 for 13. Thomas made four of Oregon's 12 3-pointers, while Washington State had four.
Jillian Alleyne had 15 points and 13 rebounds for the Ducks.
Tia Presley led Washington State (5-13, 1-6) with 16 points. Lia Galdeira scored 13 points for the Cougars.