My buddy monsterb has made a site that lists all of the known Oggcasts and a definition of what an oggcast is. From the site:
Welcome to Oggcast Planet! Here you will find links and information on high quality shows that are 100% Oggcasts. Please visit the Members page for detailed info! Some shows claim to be an Oggcast, but actually they are just Podcasts. An Oggcast is a show that offers only Ogg to their listeners and not patent encumbered codecs like mp3. What is an Ogg you ask? Ogg Vorbis is a completely open, patent-free, professional audio encoding and streaming technology with all the benefits of Open Source. Oggcast Planet fully supports the PlayOgg campaign founded by the Free Software Foundation.
Find all of your Freedom loving Ogg goodness at http://oggcastplanet.org/index.html