Why couldn't Goku use the Nimbus to rescue the Ox King from the fire?
It was most probably because Ox King did not have a pure heart, and so would not have been able to ride the Nimbus. Evidence for this is seen during the first meeting of Goku and Ox King, when rumors that Ox King murdered those who approached his castle for treasures (out of greed, of course) was confirmed by him when he admits his wrongdoings in front of Master Roshi.

Anyways, it is filler content and not part of the original manga, so no "official" explanation exists.