A .OGG audio file is a file which contains audio samples compressed with the Vorbis audio encoding algorithm, which is a very good, open source audio codec.
Vorbis compression is comparable (if not better) than MP3 one, but unfortunately very few portable players support it, so if your player only supports MP3 files and you got some Vorbis files, you need to convert them from Vorbis to MP3.
The good new is that FreeRIP OGG Converter convert OGG to MP3. The bad new is that such conversion will decrease the audio quality.
This happens because both MP3 and Vorbis use "lossy" compression schemes, so they throw away part of the audio information in order to decrease the size of the output file: when you convert a full quality audio source (like a CD audio or a Wav or a Flac file) to Vorbis, the compression will throw away some information, and again when you convert Ogg to MP3 the encoding process will throw away other information.
So if you can find the original CD it's better that you convert it directly to MP3, but if you can't find it, you can use FreeRIP OGG to MP3 Converter to convert Vorbis files to MP3: use View->Converter menu option to set FreeRIP to "Converter" mode.
Once FreeRIP is in Converter mode you can use File->Add files to convert... menu option to build the list of Vorbis files to convert. Alternatively you can use Windows drag&drop: drag audio files from a folder and drop them onto FreeRIP window.
Once the source file list is complete, use Rip menu to convert them in the chosen output format. E.g. select Rip->Convert selected tracks to MP3 to convert to MP3.
If you have multiple format audio files to convert to MP3 (or another format), you can add them all to FreeRIP Converter interface and bulk convert them.