Carlos Emory added 14 points for the Ducks (21-8, 12-5 Pac-12), who shot 61.5 percent in the second half to overcome a 41-36 deficit at halftime.
Joseph and Sim each scored 13 in the second half. Joseph finished 11 for 16 from the field, and Sim was 6 for 10 and made a season-high five 3-pointers on seven attempts.
In his last two games combined, Sim is 16 for 24 from the field with nine 3-pointers and 44 points.
Carlon Brown led the Buffaloes (19-10, 11-6) with 20 points.
Oregon is tied for third place with Arizona in the Pac-12 with only a game against Utah left in the regular season. The Pac-12's top-four teams earn first-round byes in next week's conference tournament.
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