There is no SMI-S option for McData.
Do the McData datasources ever inventory successfully?
If yes - what is the ping time between OCI and them? It may be that WAN issues are a problem. It may mean that deploying a Remote Acquisition Unit may help, or it might be possible to bump up retries within the datasource.
consult the support matrix, and ensure your firmware is supported. Some firmware fundamentally have wonky SNMP stacks, and are unsupportable
Use the snmpwalk.exe to validate that your OCI server can communicate via SNMP to the McData switch with the same community string you are using.