There is nothing special you need to do - ogg files work exactly as wav or aiff or any other file supported by libsndfile.
It is up to you, whether to use GEN 01 or diskin, soundin or any other Csound opcode that deals with audio files.
Maybe the only thing to keep in mind - since ogg need decompressing/deconding, loading it takes a bit more time that wav/aiff, so if you need to use one file repeatedly, GEN 01 is probably the way to go.
You can use ogg in the same way you use a wav or aiff, no difference. Anywhere a wav file goes, you can put an ogg.
Prof. Victor Lazzarini
Maynooth University
Ireland