OpManager uses the first OID (hrStorageType) to find the list of fixed disks and the next 3 OIDs will be used for actual calculation only for those Fixed disks.
Refer the attached image with the SNMP response for the above mentioned OIDs. Click Settings-->Tools MIB Browser and load Host Resources-MIB and provide the device details and its community string and check responses for those OIDs.
Note:You can also check the same using the standalone MIB browser by Open CMD prompt as Admin and execute mibbrowser.bat under /opmanager/bin. Load HostResources.mib and query the OIDs mentioned above.
Out of 5 instances last three are the actual fixed disks(partitions). Others are RAM info and removable disks. So the we need to use only the last 3 instances for calculation.