Brilliant Service is the company behind the Viking OS HMD software and their PDF pamphlet goes over some of the details. The company calls smartphones outdated and their app development is in Objective-C. Their described reasons for Objective-C is that it's "elegant, fun, and easy to program." They also attribute faster app development compared to C/C++ and Objective-C being used in iOS and OS X.
Underneath the hood we found out Brilliant Service is using FreeBSD as the basis for its Viking OS platform. They are using FreeBSD and their application framework is in Objective-C using GNUstep for an open-source implementation of Apple's Cocoa frameworks.
Embedded below is a demo of the FreeBSD-based Viking OS in action on a head-mounted display.