The Opera Australia Orchestra (based in Sydney, New South Wales, Australia) is a full-time salaried orchestra, and a wholly owned subsidiary of Opera Australia. It is one of three salaried orchestras in Sydney, along with the Sydney Symphony Orchestra and the Australian Chamber Orchestra. The OAO performs almost exclusively in the Opera House Orchestra pit of the Joan Sutherland Theatre.

The OAO (and its Melbourne counterpart Orchestra Victoria) were known as The Elizabethan Sydney Orchestra and The Elizabethan Melbourne Orchestra respectively, established by the Australian Elizabethan Theatre Trust in 1967. These orchestras became the 'Australian Opera and Ballet Orchestra' in 1991, and State Orchestra of Victoria in 1986. The SOV has since been renamed Orchestra Victoria. In 2017, it was renamed the "Opera Australia Orchestra".

OAO's core strength is 53 players (compared with Sydney Symphony's 110) but is currently down to 42 (November 2020), due to Opera Australia making 16 positions redundant during the global pandemic. The company intends to replace a permanent workforce of secure jobs with a temporary workforce and insecure jobs. . Each player is selected by audition; if successful there is a trial process which may require 12 months to complete. More than 120 other musicians may join the OAO as casual musicians during a financial year; this is due in part to Occupational Health and Safety Act NSW 2000 requirements regarding noise levels associated with working in the Sydney Opera House orchestra pit.