Hi all, hope this may help someone..
Customers I deal with on occasion want to monitor OIDs on a SNMP capable device (like an environmental sensor) and will (hopefully) fire you a MIB file to look at and import into snmpget. To work out what root OID to point the MIB Browser to in snmpget, you would typically open the MIB file and either hope that it's well-documented (usually it's not from my experience!) - or spend a while reading down the file piecing together the OID dot-by-dot.
I found a free tool called the Paessler MIB Importer http://www.paessler.com/tools/mibimporter which seems to do a good job interpreting a MIB file and presenting the available OIDS for you to browse and search through. Note, Paessler makes a product called PRTG which competes against us so please don't use this software in front of the customer!
How to use:
Once installed, open and select Import -> MIB File then locate the MIB file for the SNMP device
The OIDs in the MIB will appear in a tree on the left within tables/entries (note they're in alphabetical order, not ordered by the OID number). Once you find an OID name you need you can then grab the OID in full in the Source area to the left then paste that into the MIB Browser Object Identifier field (drop some digits to the furthermost right to get more than just that one OID)