http://www.usedoboes.com
You'll want to make sure you have a left hand F on the oboe you get. Some ads will state such things, make sure you have a clear picture so that you can see for yourself. Some people have the notion that the left hand Eb key is the left F... since you can use that for forked F rather than the right hand Eb key (boggles the mind).
This is what full left hand keywork looks like:
http://www.kesslermusic.com/html/4052/leftf.jpg
The oboe you have now should have left hand keys for Ab, Eb, and B. Your next oboe will add two more unique keys for Bb and F.
The ring key aides in the Eb-E trill and is probably not as desirable as a 3rd octave key which facilitates easier access to highest E on up.
You can tell if an oboe had a 3rd octave key from the front. Look at images of oboes with and without 3rd octave keys and see if you notice something missing between the first trill key vent and the thumb key vent.
Oboes with ring keys, but no left F keys are quite old and should be avoided. Watch out for these... just because it has a ring key doesn't mean it has everything else. Even some old Lorees lack left F's.