for i in `ls *.mp3`
mv $i $(echo $i | sed s/.ogg$/.mp3/)
rename would be the easy way to do it. I never seem to want to do things the easy way however.
This is mostly an academic exercise for me. I was at a LUG last night where the presenter used a string very similar to the one you provided to perform a *.ogg to *.mp3 name change to files in a directory.
I've been banging my head against a wall trying to come up with a single string that can do it, but now I realize that it probably was a script.