Mason spent last season as defensive coordinator at Auburn but left the program Saturday, as the Tigers promoted Jeff Schmedding to replace him. Auburn ranked 27th nationally in points allowed under Mason last season.
An announcement from Oklahoma State could come later Wednesday, according to sources.
Mason, 52, will replace Jim Knowles, who left to become defensive coordinator at Ohio State. Knowles dramatically improved Oklahoma State's defense during his tenure, and the Cowboys last season ranked fifth nationally in total defense and ninth in points allowed and reached the Big 12 championship game. Oklahoma State tied a team record with 12 wins and finished the season with a historic come-from-behind win over Notre Dame in the Fiesta Bowl.
The Mason hire represents a significant commitment for Oklahoma State, which made a competitive offer to retain Knowles, according to sources.
Mason went 27-55 as Vanderbilt's coach from 2014 to 2020. He served as a coordinator at Stanford from 2011 to 2013, and he spent three seasons in the NFL with the Minnesota Vikings.