Although it would be awesome to ditch Net-SNMP altogether now that the base OpenBSD SNMP daemon has support for all of the OpenBSD-related MIBS (CARP, PF, kernel sensors), reality is that Net-SNMP still offers some features that are needed. OpenBSD doesn't have any SNMP tools (snmpwalk, snmpset, etc) so these are still required from Net-SNMP. There's also some unique features in the Net-SNMP daemon that are still useful if you want to do things like monitor BIND9 or Postfix statistics.
Here's how to run both at the same time and leverage snmpd for the OpenBSD-related MIBs and the Net-SNMP daemon for its ability to retrieve data from scripts and extend itself using loadable modules and smux sub-agents.