The development of an open source tool for electromagnetic transient (EMT) and phasor domain hybrid simulation, named OpenHybridSim, is presented in this paper. The development philosophy, program structure and implementation are discussed in detail. A socket communication based interface framework allows users to integrate this tool with different EMT simulators. An interface with PSCAD is provided as an example. Besides the conventional positive sequence transient stability (TS) simulation, a three-sequence TS simulation is also developed in this tool. A multi-port three-phase Thévenin or Norton equivalent of the external system is utilized to simulate cases involving unbalanced conditions such as unsymmetrical faults. In addition to the source code and application programming interfaces (API), OpenHybridSim also provides a graphical user interface (GUI) to enhance user friendliness. The tool is tested using the IEEE 39-bus system interfaced with a VSC-HVDC system.