'OS' was the designation of Allied convoys (together with a numerical suffix) plying the route from Liverpool, UK, to Freetown, Sierra Leone (July 1941/May 1945).

This 'Outbound South' series was suspended temporarily between September 1942 and February 1943, and from April 1943 such convoys, of which there were 131, sailed with 'KMS' convoys as far as Gibraltar.

The first of the series was OS.1 of 24 July/10 August 1941 with 51 merchant vessels and an eventual total of 24 escorts, and the last was OS.131/KMS.106 of 27/30 May 1945 with 14 merchant vessels and four escorts.

Typical of the series was OS.33, which departed Liverpool in the UK at 16.05 on 1 July 1942 with 41 merchant vessels bound for Freetown, Sierra Leone, in 'Parole'. Between 22 June and 2 July six U-boats left their various bases in France with orders to rendezvous in a position to the east of the Azores islands group and form the 'Hai' (i) wolfpack to await the arrival of OS.33. In summary, Kapitänleutnant Heinrich Zimmermann’s U-136 was sunk by the frigate Spey, sloop Pelican and Free French destroyer Léopard, six merchant vessels which left OS.33 were torpedoed and sunk, and the U-boats accounted for a further nine vessels during this patrol.

An initial 20 merchant vessels sailed in line down the estuary of the Mersey river to the end of the swept channel, where they redeployed into two lines until they were joined by eight vessels from Milford Haven, nine from the Clyde river and five from Oban, after which the convoy formed nine columns of four or five ships, which was the convoy formation. In all, therefore, 41 merchant vessels, 10 of them carrying dangerous cargoes, sailed round the north of Ireland bound for the South Atlantic. The convoy commodore was O. H. Dawson, who sailed in the centre lead vessel, the 6,601-ton Castalia for Freetown, while the masters of the 6,568-ton New Texas and 6,568-ton New Toronto acted as vice and rear commodores respectively. The convoy included one oiler, the 7,347-ton Laurelwood, to refuel the escort vessels. In the event of any ship being damaged while in the convoy, the rear ships of the columns were designated as rescue ships with orders to save life and rejoin the convoy.

The normal cruising speed was 8 kt. The Liverpool contingent had been at sea for only 150 minutes when the 545-ton armed trawler Ophelia, which was unconnected with the convoy, was in collision with the 7,173-ton Ocean Honour. The trawler had to return to Liverpool for repairs, while the merchant vessel was only slightly damaged above the waterline and was therefore able to continue with the convoy. The passage to the Azores islands group was uneventful except that on 10 July the 7,524-ton Empire Attendant finally lost contact with the convoy after breaking down for the seventh time, and was later torpedoed, with the loss of all hands, by Korvettenkapitän Werner Schulte’s U-582. At 09.00 on 11 July the vessels bound for South America (4,284-ton Shaftesbury, 4,763-ton Radcombe, 6,723-ton Sithonia, 5,334-ton Bruyere, 7,093-ton Cortona and 5,242-ton Siris) together with the 4,846-ton Consuelo and those bound for South Africa (8,826-ton Port Hunter, 9,460-ton Glenberg, 5,011-ton US Dona Aurora, 5,184-ton Norwegian Sophocles and 4,890-ton Marsdale) left the convoy to proceed independently.

It was not until 15.34 on 11 July that the Admiralty reported to the convoy the interception of a 12.58 radio signal from a U-boat giving a sighting report of OS.33. The report came too late for the vessels which had already been detached, and of these Cortona, Port Hunter, Siris, Shaftesbury, Sithonia and Empire Attendant were sunk by Korvettenkapitän Adalbert Schnee’s U-201, Schulte’s U-582, Schnee’s U-201, Korvettenkapitän Werner von Schmidt’s U-116, Schnee’s U-201 and Schulte’s U-582 respectively.

The rest of the convoy reached Freetown on 20 July.