Oshkosh's prototype is much less radical and boxy than the Karsan proposal. Really, it's just a converted Ford Transit. The hood, lights, and most of the rear box are typical Ford pieces. The grille opening is the same, too, just with a redesigned mesh insert. But there are quite a number of custom changes. For one thing, it has gained enormous rubber bumper extensions. You could drive this into a bumper car arena and no one would question it.
The cab has large extensions around the doors, and in fact, the extensions are likely all door. On the right side of the van, where the driver sits for easy mailbox access, a full-size sliding door has replaced the conventional Transit door. Part of why it sticks out so far is that there is a second sliding door behind it to access the cargo area. This means that there's nowhere to fit rails for a sliding door mechanism, so an external rail was mounted to the top of the door frame. The door on the left side also protrudes quite far, but it lacks the high-mounted sliding rail, so it may open up differently to the driver door.
Most of the back of the Oshkosh appears to be standard high-roof Ford Transit equipment with the exception of the rear doors. The normal barn doors are replaced by a roll-up rear door. This is useful because it means that the rear can be opened even with another vehicle practically on the van's bumper.
As we mentioned in the post about the Karsan, the U.S. Postal Service should be testing prototypes and getting close to wrapping up that phase of the selection process. It plans to issue final contracts by the end of the year. It's also possible that this won't be the only USPS mail truck. The organization has said it's open to having multiple types for different uses. Hybrid and alternative-fuel vans are also being considered.