OpenBSD/riscv64 is a port of OpenBSD to various 64-bit RISC-V based systems.
The current target platform is the HiFive Unmatched board.
When booting the HiFive Unmatched board, write an install/miniroot image to a USB flash drive, and boot with that drive plugged in. Make sure you also have the microSD card plugged in that shipped with the HiFive Unmatched board. The board should auto-boot the OpenBSD bootloader, which will load the bsd.rd RAMDISK install kernel.
OpenBSD/riscv64 runs on the following hardware:
Snapshots are made available from time to time, in this location as well as on a few mirrors. Here are the OpenBSD/riscv64 snapshot installation instructions as well.