 Castillo opted to pass on an opportunity to play professional baseball and instead enrolled in college at Oklahoma State University (OSU) where he was coached by the legendary Hall of Fame coach Gary Ward.While Castillo was a junior in 1987, OSU reached the championship game in the College World Series against Stanford University, losing 9 to 5 to future MLB pitcher Jack McDowell.Castillo was drafted again in the 1988 Major League Baseball draft by the Detroit Tigers organization, beginning his career there that season.