SOS is a fixed shooter, in the style of Space Invaders. The player controls a World War II-era twin rotor aircraft at the bottom of the screen, which can only maneuver horizontally. A barrage of single rotor fighters descends from the top of the screen during play, and the player must destroy these fighters before they reach the bottom of the screen. Each fighter destroyed is worth 10 points, and each fighter that reaches the bottom of the screen increments separate count by 1. If the number of enemy planes that have flown past without being destroyed reaches 100, the game ends automatically.
During play, an on-screen SOS prompt will occasionally appear on either the far left or right of the play field (accompanied by S.O.S. in Morse code). If the player can reach this flashing prompt before it disappears, they score 30 points and the missed fighter plane total decreases by 9.
Once the player reaches a score of 2000, and every increment of 2000 points thereafter, they are "treated" to a very early instance of fan service in games: an image of a bikini-clad woman in sun glasses appears on screen, accompanied by the text "COFFEE BREAK" and a four note set of descending octave bleeps.