The Old Boys' Union (OBU), founded in 1893, is run for Old Boys by Old Boys.Founded in 1893 and boasting over 8,000 members today, the OBU holds a significant place in the history and ongoing support of The King's School.

The OBU allows past students to remain engaged with their school peers, establish a broader network and stay connected with King's. Social activities such as the annual dinner, networking events, country functions and reunions provide avenues for fostering these connections. For younger members, participation in sports clubs such as the Old Boys' Rugby Club and Football Club, allows Old Boys to stay connected to the School whilst engaging in competitive sports against teams from across Sydney.

The OBU's contributions span various realms, from financial assistance for School buildings and developments, to the provision of scholarships and bursaries.