Here is a short guide explaining how to install OpenBSD in Qubes OS, as an HVM VM (fully virtualized, not integrated).
Download an ISO file to install OpenBSD, do it from an AppVM. You can use the command `cksum -a sha256 install73.iso` in the AppVM to generate a checksum to compare with the file `SHA256` to be found in the OpenBSD mirror.
In the XFCE menu > Qubes Tools > Create Qubes VM GUI, choose a name, use the type "StandaloneVM (fully persistent)", use "none" as a template and check "Launch settings after creation".
In the "Basic" tab, configure the "system storage max size", that's the storage size OpenBSD will see at installation time. OpenBSD storage management is pretty limited, if you add more space later it will be complicated to grow partitions, so pick something large enough for your task.
Still in the "Basic" tab, you have all the network information, keep them later (you can open the Qube settings after the VM booted) to configure your OpenBSD.
In "Firewall rules" tab, you can set ... firewall rules that happens at Qubes OS level (in the sys-firewall VM).
In the "Devices" tab, you can expose some internal devices to the VM (this is useful for networking VMs).
In the "Advanced" tab, choose the memory to use and the number of CPU. In the "Virtualization" square, choose the mode "HVM" (it should already be selected). Finally, click on "Boot qube from CD-ROM" and pick the downloaded file by choosing the AppVM where it is stored and its path. The VM will directly boot when you validate.
The installation process is straightforward, here is the list (in order of appearance) of questions that require a specific answer:
Whether you reboot or halt the VM, it will be halted, so start it again.
You should get into your working OpenBSD VM with functional network.
Be careful, it doesn't have any specific integration with Qubes OS like the clipboard, USB passthrough etc... However, it's a HVM system, so you could give it an USB controller or a dedicated GPU.
It's perfectly possible to run OpenBSD in Qube OS with very decent performance, the setup is straightforward when you know where to look for the network information (and that the netmask is /8 and not /32 like on Linux).