This is a mini-review since it's roughly 30 minutes long, and there are some minor spoilers.
There's not a lot to say about this one. It's a short OVA that focuses mostly on Senji and his days as a police officer before he got sent to Deadman Wonderland. It reveals some backstory, but it doesn't solve the underlying problem of Deadman's story being criminally incomplete. It's backstory and nothing else. It's got some of Deadman's trademark action, yes, and it's always fun, but this OVA isn't much at all.