Monitoring a RedHat cluster:

snmpwalk -v3 -u username -A password -a md5 hostname  \
  -l authNoPriv REDHAT-CLUSTER-MIB::RedHatCluster

Monitoring 3ware disk array

snmpwalk -v3 -u username -A password -a md5 hostname  \ 
  -l authNoPriv TW-RAID-MIB::twRaidMIB

With nagios you can use checkcommands, and custom variables for hosts as:

define command {
  command_name    check_snmp3_mib
  command_line    /usr/lib/nagios/plugins/check_snmp -U $_HOSTSNMP_USERNAME$ \
          -A $_HOSTSNMP_PASSWORD$ -L $_HOSTSNMP_SECLEVEL$ -a $_HOSTSNMP_AUTHPROT$ \
          -H $HOSTADDRESS$ -m $ARG1$ -o $ARG1$::$ARG2$.0 -w $ARG3$ -c $ARG4$ -l $ARG2$
}

define host{
  use             generic-host
  name            generic-host-company
  register        0
  parents         Internet
  _SNMP_USERNAME  username
  _SNMP_PASSWORD  password
  _SNMP_SECLEVEL  authNoPriv
  _SNMP_AUTHPROT  MD5
}

define service{
  use                 company-service
  host_name           hostname
  service_description rhcClusterFailedServicesNum
  check_command       check_snmp3_mib!REDHAT-CLUSTER-MIB!rhcClusterFailedServicesNum!1!0
}