O'Sullivan grew up in the San Francisco Bay Area and was introduced to soccer while living in La Honda.
His first team was the La Honda Bandits which played in the Northern California division of the American Youth Soccer Organization (AYSO).
O'Sullivan attended the Woodside Priory School in Portola Valley and was named All American his senior year.
Matt played three years with the Juventus Soccer Club of Redwood City during which two state championships, as well as two trips to the final eight of the McGuire Cup, were achieved.
In 1976, Seattle Sounders selected O'Sullivan in the first round of the North American Soccer League draft.