'OA' was the designation of Allied convoys (together with a numerical and sometimes a literal suffix) plying the route to the west from Southend-on-Sea on the Thames estuary (or from July 1940 Methil on the Firth of Forth) toward Liverpool for dispersal about 750 miles (1205 km) to the west of Land’s End (September 1939/October 1940).

At intervals, the 'OA' convoys would join with the complementary 'OB' convoys from Liverpool to make the passage to Gibraltar. In this case both the 'OA' and 'OB' convoys would be designated 'G' and become one of the 'OG' series. Even when dispersing in the Atlantic, the 'OA' convoy would combine with the 'OB' convoy if they met at sea.

There were 226 such convoys, and from 3 July 1940 they joined 'FN' or 'OB' convoys for movement through the South-Western Approaches.

The first of these convoys was OA.1 of 7/9 September 1939 with three British merchant vessels in the form of the 1,931-ton Adjutant, 8,006-ton Gloucester Castle and 6,734-ton Pacific Exporter, escorted by the British destroyers Acasta, Amazon and Antelope. The last was OA.234 of 24/26 October 1940 with the 1,174-ton Norwegian Elna E, 3,829-ton British Eskdene, 691-ton Turkish Kilye, 1,260-ton British London II, 1,014-ton British Medway Coast and 852-ton Norwegian Skarv, escorted by the the light anti-aircraft cruiser Cairo, sloop Rochester and corvette Primrose. OA.234 made rendezvous with OB.234 on 26 October.

A notable example of the 'OA' series was the OA.178 convoy, which departed the Thames estuary on 3 July 1940 for the English Channel before dispersing to a number of transatlantic destinations. The convoy gathered off Southend-on-Sea on the coast of Essex as 35 merchant ships escorted by the corvette Clarkia, passed westward though the Strait of Dover during the night of 3/4 July, and by the afternoon of 4 July was in the English Channel, some 20 miles (32 km) to the south of Portland Bill. This took place during the period known to the Germans as the Kanalkampf (channel fight), which was the first stage of the the Battle of Britain, and at about 13.00 on 4 July two Gruppen of Junkers Ju 87 dive-bombers from Oberstleutnant Oskar Dinort’s Stukageschwader 2 attacked the convoy in a succession of six-aircraft waves. There was no Allied air cover and the attack lasted about two hours.

In this attack the 4,952-ton British Dallas City was sunk and the 5,225-ton British Antonio, 7,178-ton British Argos Hill, 4,019-ton British Briarwood, 5,812-ton British Eastmoor and 2,254-ton Danish Lifland were damaged. Antonio rescued 67 survivors from Dallas City.

Some of the convoy’s ships took shelter in Portland harbour, where again they came under German air attack. Also in Portland were a number of other ships, which were also hit in the attack. The anti-aircraft ship Foylebank was severely damaged, and sank on the next day with the loss of 176 lives. The tug Silverdial was also sunk, and the cargo ships City of Melbourne, East Wales and William Wilberforce were damaged.

In the earlier attacks bombs had damaged Antonio's rudder and the stanchions supporting her propeller shaft, and the vessel had to reduce speed and therefore did not reach Portland until about 23.00. The continuing air raid prevented her from landing the survivors she had rescued until 12.00 on 5 July.

At dusk those members of the convoy which had remained at sea were 13 miles (21 km) to the south of Portland when they were attacked by aircraft and S-boote. Two Dutch vessels, the 5,255-ton Britsum and 1,796-ton Deucalion, succumbed to air attack but suffered no casualties, while the 3,526-ton Estonian Kolga was sunk by S-19,with one casualty, and the 4,343-ton British Elmcrest by S-20 with 16 casualties; another ship, Hartlepool, was damaged.

On 5 July the convoy’s lone escort was reinforced by the destroyer Broke, which remained with the convoy until 6 July and its dispersal in the South-Western Approaches. It was the severity of this convoy’s losses that led to the decision that all subsequent OA convoys be re-routed from Methil in Fife around the north coast of Scotland to avoid the English Channel and its threats of air and S-boot attack.