The Orbiter Multiplayer Project (OMP) is intended to bring multiplayer features to Orbiter. This is one of many attempts to accomplish the goal on both LANs and WANs and incorporates the idea of a single "universe" server.
The referenced website hosts the source-code repository. A public server link can be found there, too.
The download link always points to the newest version of the source-code and binaries in a ZIP archive. Please note that there is an additional folder in the ZIP root labelled like "OMP-OHM". Within this folder you will find the usual Orbiter addon layout (there are also some additional ".hg*" files that are not needed for the software to work). For further installation instruction, see here.