z/OpenBSD -- Wibni?
There is no z/OpenBSD. But there should be.
There is no OpenBSD cross-compiled to run native on the instruction set prescribed by the 390 Principles of Operation and hosted by the z/VM Operating System which is unto PC server virtualization software like Sauron was to Saruman in Lord of the Rings . (Saruman thought he was competing with Sauron, but he wasn't playing in the same league as the old devil.)
Linux is there on the mainframe, has been there, for almost a decade now, and used with success. Apparently OpenSolaris is to follow. I think the progression is in the wrong direction
Linux and OpenSolaris are designed to be user-friendly operating systems that wrestle PC users away from Microsoft. That's not IBM's mission. That's not the data center's mission chez the Customer.
IBM wanted an open source Unix to run as a virtual machine, and Linux is that, and it is there. But OpenBSD, once ported, would have some real advantages.
Mainframes running z/VM are sort of like multi-million dollar embedded controllers. OpenBSD, which runs all the same open source toys that Linux does (and builds them easily onsite due to a ports system more mature than that of Linux) is designed more like a z/VM guest than Linux is.
The best feature today of Linux's execution model is its multiproc kernel threading. Really, this is not what z/VM wants in a guest, a complicated and competing threading model. z/VM for performance wants a conservative implementation of Unix.
OpenBSD is not as big a hog as Linux or OpenSolaris. z/VM Unix guests shouldn't demand resources if it can be helped. Virtual images are there to run some application, usually one application, no more focussed on interactive loads than other data center virtual machines.
And OpenBSD is more secure and more securable than Linux because it's simpler than Linux and because security is designed in, not bolted on.