In principle, if the provider does support full-virtualization and has a debug console, one could install any operating system of their choice, manually.

It may be possible even without one, if you can somehow figure out how the boot process works. The most crazy successful attempt I made was performing deletion of the entire Linux root filesystem with a busybox shell running in RAM over SSH, and replacing it with to another unsupported distribution, then rewrite the grub.cfg, configure the network settings and trigger a clean filesystem sync and reboot via proc. Finally pray you'll see the server coming online again soon. It worked.

But providing a template is all it needs for the provider to support OpenBSD, unfortunately it is still unavailable. Is it that hard to provide a template?

Vultr has it listed now officially. Don’t think you even need to upload the iso anymore.

Can confirm OpenBSD works fine on Vultr. No weird install problems.