The Orient Steam Navigation Company, also known as the Orient Line, was a British shipping company with roots going back to the late 18th century. From the early 20th century onwards, an association began with P&O which became 51% shareholder in 1919 and culminated in the Orient Line being totally absorbed into that company in 1966.

 Each company had a vessel sailing from England to Australia every two weeks, resulting in a weekly service of fast mail ships. This was at a time of rapid expansion for the Orient Line, with a succession of larger ships being built. All had names starting with 'O', such as Otway, Osterley, Orsova, Otranto, Ortona and Orvieto

after the war. Three new ships of 28,000–29,000 tons entered service: Orcades (1947), Oronsay (1951) and Orsova (1953), matching in speed and size the three new postwar ships for P&O (Himalaya (1949), Arcadia (1954) and Iberia (1954)). All had higher speeds that allowed them to reduce the sailing time from England to Australia by eight days to 28 days

The Oriana was the last ship ordered for the Orient Line, and the last to fly the Orient Line flag. P&O and Orient Line were formally merged in 1960 to form P&O-Orient Lines. In 1964 the Orient Line colour scheme of corn-cream coloured hulls was discontinued in favour of P&O's white livery, and Orcades and Oronsay transferred to the P&O fleet. The name Orient Line was dropped altogether in 1966 when Orsova and Oriana were also transferred to the P&O fleet. Symbolically, the last, largest and fastest ship of the Orient Line, the Oriana, wore the Orient Line flag for her final voyage prior to retirement in March 1986.