I am trying to edit a Theora video in Premiere Elements 10. I installed the Xiph OpenCodecs package which allowed me to view the video in WMP, but it still wouldn't import into PRE. I then tried installing QuickTime and the XiphQT package, but still no joy. However, QT wouldn't play the video so maybe this didn't work. I notice that the Xiph website has not been updated since 2009, so maybe these are out of date.
Currently it doesn't appear possible to directly import Ogg Theora video into Premiere Elements 10, however I did manage to convert the video to DNxHD/VC-3 so that PRE would handle it nicely (previous conversions had behaved very badly)
ffmpeg -i out-48.ogv -s 1920x1080 -r pal -b 36M -threads 2 -vcodec dnxhd -acodec libmp3lame out-48.mov