Switching from SNMP v2c to SNMP v1 has the effect, OpenNMS will use get requests with one (1) variables per request. It you use SNMP v2c by default bulk requests with 10 variables per PDU to reduce communication overhead and be more efficient. If you try to verify if the Mikrotik SNMP agent can deal with SNMP v2c you can use the snmpbulkwalk or snmpbulkget CLI tool to simulate a similar behavior than OpenNMS. The number of variables OpenNMS requests from a device can be configured per IP address with the "Configure SNMP by IP" and the "max vars per PDU" setting.

http://forums.cacti.net/viewtopic.php?f=12&t=25349&start=90

Listen to me...configure opennms to use v1 and you're done!
Also if you use storeByGroup=true remove Octet String from collection.

All the best!

Mikrotik works fine with all another nms'es

I was able to run the collection in opennms 16.0.4 using snmp v1. This is a workaround sure.
you can also try to upgrade Router OS software. The probem is also related to Mikrotik Snmp Agent not conform to
SNMP version2.

Closing issue with words "use snmp v1" is not the fixing of a problem.