I am leaning for an avionics exam and I encountered a question asking for the meaning of F, S and O . They appear to be "Characteristic speeds", which are calculated by the flight management system (FMS):
F speed is the minimum speed at which flaps should be retracted from CONF 3 or 2 to CONF 1+F.
S speed is the minimum slats retraction speed, i.e. the minimum speed at which a clean configuration should be selected.
GD speed is the engine-out operating speed in clean configuration. In other words, it corresponds to the speed that allows the highest climb gradient with one engine inoperative in clean configuration.