An optical navigation system comprising: a control circuit; and an optical sensor, comprising a plurality of pins, wherein the optical sensor is configured to sense optical data and configured to compute motions according to the optical data. The optical sensor outputs the motions to the control circuit via a complex pin among the pins in a first mode. The optical sensor outputs data other than the motions to the control circuit via the complex pin in a second mode. The present invention also discloses an optical navigation system which shares a pin for receiving different types of data.