Yes. I run my mail server on a Raspberry Pi 4 on OpenBSD. Aside from having to flash an SD card (and having the install program install the sets to… itself) everythhing works exactly as you expect on any other OpenBSD install. You can’t really stray far outside the list of supported hardware unless you plan to roll things yourself, however. Eg, I’ve discovered that Waveshare carrier boards for the CM4 are hit or miss. NetBSD has much more comprehensive support for Raspberry Pis and the various accoutrements.

Pretty excited about Pi 5 support!