His former team made the announcement that Matt passed on Saturday. The team did not reveal the cause of death.

Matt pitched for Oakland for 7 seasons. He received some notable awards ... 1978 All-Star rookie, 1980 American League Comeback Player of the Year. In addition to Oakland, Matt pitched for the Cardinals, Cubs, Astros and Yankees.

Matt was on the cover of Sports Illustrated in '81 with the other "Five Aces" ... 4 other members of the A's.

After baseball, Matt ventured into reality TV with his then-wife, Jeana ... they were cast members on "Real Housewives of Orange County." They married in 1984 and separated 20 years later. Despite the separation, they continued living together during the early seasons of "RHOC."

Matt had his share of legal trouble. He racked up 2 DUI's ... the second of which earned him a 1-year jail sentence. Interesting ... Jeana testified on Matt's behalf, even though their marriage was over, asking the judge to give him probation.

It's especially tragic for the family ... a few weeks back their daughter, Kara, had a son who died during childbirth from complications ... shoulder dystocias and a compressed umbilical cord.

Kara posted a tearful message ... "Daddy, please take care of my son. Teach him the circle changeup and how to find forever friends. You're on grandpa duty in heaven now. Xoxo, Hammerhead."

Matt leaves behind 3 kids. He was 64.