At the western side of OGC you'll come upon a small shack with a diamond briefcase inside it. To get inside the shack, jump onto the large rock sitting nearby and jump onto the roof; once there you can use the ladder to gain entrance and loot the briefcase.
A small path leads from the southern side of OGC onto the other side of the ridge. Along the path you'll come across some ammunition and a diamond briefcase. The latter contains 2 diamonds.
The briefcase here is located on the roof of the building furthest to the west at "Private Property". To get to it, jump onto the crates that are located between the two small buildings, and from there onto the roof. Move across the ladder to loot the briefcase.
Inside a half-buried bus in the south-western part of the region (between the safehouse and the checkpoint) you can find a diamond briefcase. Right next to the briefcase lies a Jackal Tape.
In the south-eastern corner of the region you can find a small shack where the road ends. Inside you can find a diamond briefcase along with a golden AK-47 and some ammunition.