IIRC it was Suma or Maruk who said you should not compress (with pbo compression) the *.ogg files because it will infact increase their file size...
I recall that too. Based on the fact that OGG is already compressed, not much good can be done to it by trying to compress it more. A quick test using MakePBO 1.1 for a 98 KB OGG file gives me a PBO of 109 KB (compressing OGG files on). With WinRAR I get 97 KB and with Windows XP I get 97 KB.
So what's the profit if compressing OGG results in OFP having to do extra work with the file and there is no compression but you get bigger PBO's?
Leave them uncompressed.
Instead - like THobson is explaining - I would bring the quality of the OGG files down to a level where ingame sound quality is still acceptable. You can get significant file size drops and still keep most of the 'hearable' quality of your sounds.