Patricia Bright scored 14 points and Alyssa Martin added 13 for the Beavers (18-9, 8-8 Pac-12).
Amanda Johnson, in her last home game for Oregon (14-14, 6-10), had 13 points and 10 rebounds.
Oregon State's victory was the third straight against Oregon, the Beavers' longest winning streak against the Ducks since also winning three straight in 1993-94.
Oregon State trailed 29-22 at halftime and 40-32 nearly nine minutes into the second half before Bright sparked an 18-1 run that put the Beavers up 50-41 with 5:17 remaining.
Bright scored six of the first 11 points during that stretch and Marchbanks finished off the victory by going 12 of 14 from the free-throw line in the final 4:30.
Notes: Saturday's game was the final game at Matt Arena for seniors Jamin Holliday, Nia Jackson and Amanda Johnson... Oregon's one-time 10-point lead was its largest blown lead of the season...the loss also marked the second time this year Oregon blew a halftime advantage...Oregon State has now defeated Oregon in three consecutive meetings for the first time since Jan. 13, 1994...the Beavers also sweep the season series for the first time since 2009...Jasmin Holliday played in her 121st career game on Saturday, ranking fifth all-time at Oregon...Nia Jackson's lay-up at the 18:58 mark of the first half scored the 1,000-point of her career. She is 24th player in the history of Oregon women's basketball to score 1,000 points...Amanda Johnson recorded the 29th double-double of her career with 13 points and 10 rebounds, ranking fifth all-time at Oregon...Johnson's also recorded her 72nd game scoring in double figures, tying with Shaquala Williams for seventh all-time...Oregon State defeated Oregon in Eugene for the first time since a 61-42 victory on Jan. 31, 2009.