Well if the data you are getting is from Extending the Red Hat MIB, then you should have control over the script. If the script returns a string that can only have a list of possible values, you can instead return a numerical index. Many systems do this, where they will return for example a status number from 0 to 3, where 0 is ok, 1 is warning, 2 is critical, and 3 is unknown.
Also you can do a command script that will query your Extended OID and do whatever logic is needed.
I.E.
#!/bin/bash
TheVer=$1
TheCom=$2
TheIP=$3
TheOID=$4
TheCheck =`snmpget -"$TheVer" -c"$TheCom" "$TheIP" "$TheOID" | awk -F 'STRING: ' '{print $2}' | sed 's/"//g'`
the call would look like this in a command template:
<your scritp here> ${dev/zSnmpVer} ${dev/zSnmpCommunity} ${dev/manageIp} .1.3.6.x.x.x.x.x.x <- your OID
* $TheCheck now holds your string value, the rest of your script can do any logic you need, then return the result in nagios format.
Manuel