With jobs ranging from washing dishes to working in hospitals and doing research for the International Association of Fire Fighters, O'Leary then, as a young man, served in the Army Reserve and taught in public schools.

O'Leary's educational career lead him to Massachusetts Maritime Academy where he currently teaches as a tenured professor along with an adjunct position at Cape Cod Community College.

O'Leary was elected as a Barnstable County Commissioner in 1987 and took a role in forming the Cape Cod Commission, a regional planning authority for smart growth started in 1990.

O'Leary's work on the Cape Cod Land Bank, which focused on environmental and open space preservation was also subsequently used as a model used by communities across Massachusetts to this day.

O'Leary won election to the Massachusetts State Senate in 2000, becoming the first Democrat in his district elected in 40 years and eventually spearheaded the nation’s first ocean management legislation, which allows for harmonious conservation, industry and development in ocean waters.