Organized in 1960 by Capt. Gregorio S. Oca, the union occupied a small room in a building at Port Area, Manila, where all administrative functions including the membership requirements were processed. In 1972, it became an affiliate of the International Transport Workers’ Federation (ITF). 1974 saw a growth in membership that warranted the office to move to the bigger offices of the AWU-PTGWO at 13th and Boston Streets, also in the Port Area. AMOSUP now has a membership of over 75,000 ratings and officers.

On February 23, 1983, the AMOSUP SEAMEN’S CENTER in Intramuros, Manila was inaugurated on the occasion of the 9th death anniversary of Roberto S. Oca, Sr..

The four-story building on a 1,500 square meter lot is now the union’s hub of operations where all programs are centrally administered.

It houses the Executive Officers, the Agreements Unit, Welfare and Mutual Benefit Plan (WMBP), Placement Services, Records, the Legal Affairs, the Provident Fund and Administration & Finance Offices. It also houses a huge-capacity Convention Hall and a Maritime Library.