In general you shouldn't convert between one lossy format (like Ogg Vorbis) and another (like MP3). The problem is they both throw away a certain amount of data, but they do it in a different way, so you end up with a much lower quality file in the end.
If you have the CD, or the data in another lossless format (like Flac or Apple's lossless format), then you can convert that into MP3.
If you really have to convert .ogg files to .mp3, then you can use the "oggdec" executable to create a .wav file:
Then you can use any MP3 encoder to create the .mp3 files.