The Old Boys' Union opens doors to a vibrant community and a host of enriching experiences. 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.

In addition to its tangible support, the Old Boys' Union plays a pivotal role in the School's governance. Through the election of five members to the School Council, it ensures the preservation of the institution's ethos and traditions.

"The School is built upon its Old Boys; they are its strength, its safeguard, its endowment, its advertisement." Reverend Percival Stacy Waddy, Old Boy 1893, Headmaster 1907-1916. Since The King's School opened in 1832, its Old Boys have had an interest in maintaining contact with each other and supporting the governance and growth of the School. As early as 1853, Old Boys Dinners have been held, and Old Boys were instrumental in reopening the School in 1868.

On 21 June 1893, a group of Old Boys, led by Archdeacon William Gunther (1857), met at the Old School in Parramatta and established The King's School Old Boys' Union. The formation of the OBU was supported by the then Headmaster, Rev Dr Edward Harris.

Since its formation, the OBU has been led by a President and Committee of Old Boys, from a range of year groups. The OBU has held regular events for Old Boys, with the Annual Dinner being a highlight of the year.

The OBU exists not as a fundraising organisation, but to foster the mateship and memories formed during their members' attendance at The King's School. On 21 June 1993, the OBU celebrated its centenary with a re-enactment at the Old School of the original meeting that founded the OBU in 1893.

In 2018, the OBU celebrated its 125th anniversary with a gala dinner and dance held at The Trophy Room at the School. As the custodians of the traditions and standards of the School, the OBU has supported sons of Old Boys through bursaries, supported Old Boys through regular events and gatherings, and supported the governance of the School with the appointment of Old Boy Governors on the Council of The King's School.

The King's School Old Boys' Union is one of the oldest alumni associations in Australia and all Old Boys of the School are eligible for membership.