In the frame of the Call for Ideas “H2020-ESA-038 GNSS Evolutions Experimental Payloads and Science Activities”, this activity targets the development of an experimental payload for potential embarkment on-board of two Galileo satellites. The experiment supported (Optical Time Transfer Experiment - OTTEx) is based on optical technologies, i.e. optical frequency reference systems, Optical Inter-Satellite Links (OISLs) and optical frequency combs. The proposed payload shall validate optical frequency references and optical inter-satellite links for time synchronization, ranging, and data dissemination. In addition, OTTEx can be expanded to perform a MEO-to-ground Quantum Key Distribution (QKD) test.