Documentation credited by youarebritish:

Object Brush (obr) files are used in Cyprus and Mother Base to place grass, bushes, and other misc semi-procedural debris. There’s a corresponding _objBrush.fox2 which sets up data such as what fmdl to place. I believe v2 and v3 are components of a location, rotation, or scale but am not sure just yet. I also believe v5a is some kind of index, but I’m not sure into what yet.

.obr files are essentially arrays of transforms and rotations where objects(such as grass) are placed within the map.

Theories

Working theory is that the file types tre, htre, obr, obrb, lpsh, and atsh were designed by the same engineer. Reasoning behind this theory is due to the high-level structure that these formats use, are not found in other filetypes. fstb and twpf are the same way, however hold different structure.

When placing terrain decorations, the shadows are not included in the file.(?) It’s not known for sure what’s in the files besides coordinate arrays.

See also: OBRB