OBR is a file format that holds presets of Sauerbraten map geometries.
This is an incomplete overview and may contain misinterpretations, feel free to correct or add more information. :)
Example of an uncompressed .obr file:
4f45 4252 0000 0000 f001 0000 1002 0000 0002 0000 // header
0100 0000 0100 0000 0100 0000 1000 0000 0000 0000 // selection setup
0000 8083 8043 5050 8080 8080 8080 0100 0100 0200 0300 0400 0500 // first "cube"