Convert MP3 to Vorbis OGG format using ffmpeg.

ffmpeg -i input.mp3 -c:a libvorbis -q:a 4 output.ogg

If you want multiple files, use a loop.

for f in ./*.mp3; do ffmpeg -i "$f" -c:a libvorbis -q:a 4 "${f/%mp3/ogg}"; done