From the beginning, the SOS distress signal has actually been a continuous sequence of three-dits/three-dahs/three-dits, all run together without letter spacing. In International Morse Code, three dits form the letter S, and three dahs make the letter O, so "SOS" became an easy way to remember the correct order of the dits and dahs. In modern terminology, SOS is a "procedural signal" or "prosign", and the formal way to write it is with a bar above the letters. It should be written SOS.