SOS is the commonly used description for the international Morse code distress signal. When captured by the Travelers, Lt. Colonel John Sheppard modified a damaged Subspace communication array to send a SOS message through subspace using morse code. (SGA: "Travelers")
Following the Drone attack on Destiny and on the Novan colony during which the gate was destroyed, Eli Wallace attempted to use the Stargate's subspace connection to send a message to the Destiny. Lt. Matthew Scott figured out the simple action of turning the connection on and off acts as an SOS, though not the actual SOS itself. (SGU: "Common Descent")