The thing about the BSDs, and especially OpenBSD, is that their MAN pages are phenomenally good. Documentation on their respective websites covers most of the rest of what you need. If you want to really nerd out, I highly recommend all of the No Starch Press BSD books by Michael W Lucas (the Absolute BSD books are a good place to start), but they are entirely supplemental.
As the other poster said, the man pages are great, but OpenBSD's homegrown daemons are also all very similar in style. A daemon ${service}d and a control interface ${service}ctl (ie. ntpd/ntpctl ripd/ripctl ospfd/ospfctl relayd/relayctl pf/pfctl etc). The control interfaces are all designed to work similarly to each other, so they are very intuitive once you learn one. It's also worth noting that most services have separate manual pages for the daemon, the control interface, and the configuration file. I've also noted that many tools are very similar to Cisco IOS equivalents.
