On the Cowboys' depth chart released earlier this week, Mosley had been listed as the starting tight end for Saturday's season opener against Georgia.
According to court documents, Mosley is under a restraining order that was filed last week in Payne County Court by Kayla Elizabeth Starks. It is undetermined if that situation and Mosley's abrupt departure are connected.
"Jamal Mosley is leaving our football team for personal reasons," Oklahoma State coach Mike Gundy said in a prepared statement. "He has been a good teammate and we wish him the best. We will have no further comment on the matter."
Mosley was charged with misdemeanor marijuana possession on June 17, although Gundy told reporters earlier this summer that Mosley was considered to be in good standing with the team.
Mosley, a 6-foot-4, 254-pounder from Memphis, Tenn., produced five catches for 57 yards in limited playing time in 2008 as a freshman.
Wilson Youman, a sophomore who started four games for the Cowboys last season, inherits Mosley's starting position on Oklahoma State's depth chart. Youman produced one reception for four yards last season.
Tim Griffin covers college football for ESPN.com. You may contact him at email@example.com.