From Project::OSiRiON wiki

One of the goals of Project::OSiRiON is to create a game world that is easy to adapt and extend. Most of the world description is read from plain text ini files which can be edited with basically any text editor.

Editing the world The world is completely defined server-side. If you run your own server, you can offer a customized world and players will automatically join your modified world.
Creating skyboxes To give the world a more realistic appearance, each zone can be rendered with its own sky. Spacescape is a program that offers a convenient way to create skyboxes.
Creating 3D models New ships, stations and other objects can be added by creating new 3D models, the game can currently use 3D models in the ASE format, the WaveFront OBJ format or the Quake 3 MAP format. The latter can edited with the radiant editor.
Textures and materials 3D models use textures and materials, Project::OSiRiON uses material scripts to provide special texture features like player color or engine color to modellers.
Resources Links to tutorials, texture and model resources, and other related nformation.