I just upgraded a couple of machines to OpenBSD 4.9 and noticed the hrSystemProcesses OID was not being returned by Net-SNMP 5.6.1 (from the 4.9 ports/packages collection) .

joel@theta:~% snmpwalk -v2c -c public theta .
SNMPv2-SMI::mib- = No Such Instance currently exists
    at this OID

I know for sure this worked on OpenBSD 4.8/Net-SNMP

Turns out there is a bug in Net-SNMP 5.6.1 (bug 3166568) that's causing this. It's been fixed in their SVN tree. If you download this patch, place it into your ports/net/net-snmp/patches/ directory and recompile the port, you'll be good to go.