Chosen because its morse code sequence (· · · — — — · · ·) was easy to remember and recognize even through interference. Many mnemonics and backronyms were later formed from the sequence.
Usage notes 
The code SOS is normally only used in text transmission; for voice communication, mayday is used. The sequence is normally transmitted run together without any letter spacing in between, so it is technically a single unique code rather than a series of three letters.
See also 
Etymology 1 
From the letters represented by the signal, chosen as a sequence that is easy to recall and transmit (· · · — — — · · ·); it is not, as is commonly believed, an abbreviation for "save our souls", "save our ship" or any other phrase.
SOS (plural SOS's)
- The conventional Morse code call made by a ship in distress.
- The keel has been scuppered — send out an SOS.
See also 
Etymology 2 
Etymology 3 
SOS (plural SOSes)
- (games) a children's game involving placing S's and O's on a grid, and collecting points by creating an "SOS" sequence, played with alternating turns by opponents.