Sound Converter is a graphical tool that uses gstreamer as its back end. It can read Ogg Vorbis, AAC, MP3, FLAC, WAV, AVI, MPEG, MOV, M4A, AC3, DTS, ALAC, MPC, Shorten, APE, SID and can convert them to WAV, FLAC, MP3, AAC, and Ogg Vorbis files. The nice thing about the software is that you can convert entire directories at one time.
Simply, open the preferences window and choose "MP3 .mp3" from the format drop down list. Then add your .ogg files and click convert. That's it, you're done.