N'diaye played the first nine games before a left shoulder injury sidelined him the rest of the season. He averaged 3.6 points and 2.2 rebounds.
Tinkle, the son of coach Wayne Tinkle, broke his right wrist in the sixth game of the season. He was averaging 20.2 points, 8.3 rebounds and 2.3 assists.
N'diaye will be a redshirt senior and Tinkle will be a redshirt sophomore next season.
"I'm happy that Cheikh and Tres got their years back," Oregon State coach Wayne Tinkle said in a statement. "It's why the rule exists and we're sure that both student-athletes will take advantage of it from an academic standpoint as well as athletically."