I agree re: OpenSNMP. It appears as though net-snmp has several BSDish license on it; is this compatible with OpenBSD's license requirements for inclusion into the main tree? SNMP monitoring capability is required for most enterprise servers, which would make it a reasonable inclusion to me. I freely admit that others' mileage may vary.
What exactly do you want to monitor on an OpenBSD system that the standard MIB's dont already provide? Also, net-snmp can fairly easily be extended to provide pretty much any data you want.
--- Lars Hansson