First, install any vendor custom mib: On RHEL, CentOS, and related, copy the vendor MIB file to /usr/share/snmp/mibs/ , with a .txt extension; rename the file to be the same as the MIB name, e.g., NETAPP-MIB.txt
(in the examples below, supersecret is the community string, and myhostname is the hostname / ip addr of the target.
To get a specific value, based on textual MIB name:
snmpget -v 2c -c supersecret myhostname NETAPP-MIB::dfFileSys.1
To walk, with textual/name (not numeric) descriptions/OIDs:
snmpwalk -v 2c -c supersecret -m +ALL myhostname 1
...the one at the end means "give me everything". THe "-m +ALL" means "use all MIBs".