Oberkommando Der Wehrmacht

It went on a good sale at an antique store I frequent, so I picked it up. It's an Oberkommando Der Wehrmacht rubber stamp. The rubber is quite cracked and warped.

I'm just wondering if it's authentic and what its original purpose exactly was?

The second image I stamped and then smooshed the paper up against the rubber as the cracking has lead to its not being able to put a full stamp on paper (hence why you see more than what was originally intended).