Just to add some additional detail: the control plane (Omicron[0] -- named before the COVID variant!), the user-level portion of the hypervisor (Propolis[1]) and the storage subsystem (Crucible[2]) are all open source. And while our focus has been building them for our own product, we know that they are at least usable outside of Oxide![3][4]
The host is using Illumos and bhyve for hypervisor stuff. I don’t work on that part of the stack personally so that’s the most detail I can give you.