I recently switched my RockPro64 over to OpenBSD and so far everything works nicely with it. I had trouble getting it to boot at first, but it was my fault for not fully reading the installation instructions[1], and assuming that I could simply `dd` the provided miniroot75.img to an SD card and boot. In fact you also have to write bootloader firmware to certain constant addresses on the SD card, depending on which board you use.
I also would have been lost without a UART USB adapter. [1] https://ftp.openbsd.org/pub/OpenBSD/7.5/arm64/INSTALL.arm64
