The OpSTAR Phase 0 study is a first step in the development of an in-orbit Testbed for payloads enabling bi-directional Optical Links for time-transfer and ranging. The Testbed product comprises two identical satellites, each carrying a payload hosting the core technologies, as well as two ground stations, the ground processing facility and equipment. It will be designed to execute a number of key experiments centered around these core technologies with a focus on related applications. The satellite payloads enable simultaneous Time transfer, Data Transfer and Ranging between the satellites and the ground stations via bi-directional optical links. Such technologies and techniques are key enablers for miscellaneous future missions and therefore have a large commercial viability as products in case of successful demonstration