The "Oberon" class was based heavily on the preceding "Porpoise" class of submarines, with changes made to improve the vessels' hull integrity, sensor systems, and stealth capabilities. Eight submarines were ordered for the RAN, in two batches of four. The first batch was approved in 1963, and the second batch (including "Orion") was approved during the late 1960s, although two of these were cancelled before construction started in 1969, with the funding redirected to the Fleet Air Arm. This was the fourth time the RAN had attempted to establish a submarine branch.