From interpretation of the signal (· · · — — — · · ·), chosen merely as a sequence that is easy to recall and transmit, as normal Morse letters.
It is not, as is commonly believed, an abbreviation for "save our souls", "save our ship", or any other phrase.
Meanings
The conventional Morse code call made by a ship in distress.
"Look at wireless. Look at the S. O. S. call at sea. Is that not a benefit to mankind?" 1925 July – 1926 May, A[rthur] Conan Doyle, “(please specify the chapter number)”, in The Land of Mist (eBook no. 0601351h.html), Australia: Project Gutenberg Australia, published April 2019:
At around 5:30 a.m., crewmembers of a JCG patrol boat received an SOS from the 106,726-ton freighter Anangel Courage. The patrol vessel and a JCG plane found the freighter on the high seas about 67 kilometers away from Uotsuri Island, one of the Senkaku Islands, which are also claimed by China. 2016 August 11, “Chinese fishing boat collides with Greek freighter near Senkakus, 6 rescued”, in The Mainichi, archived from the original on 12 Aug 2016:
The crew sent a frantic SOS as they realised the scale of the disaster.