Indeed, Tony would usually pull out the riff and chord progressions and Ozzy would say things entirely spontaneously (and often nonsensical) to get a feel for the vocal, then usually him and Geezer would work through and create proper lyrics - I am tempted to believe however that Geezer played a huge part in Paranoid since Ozzy didn't actually know what paranoid meant
In cases where the words came first and music second, I believe Geezer was usually the sole lyricist.
I think you could also argue to an extent Ward's drumming was an integral part of the music, ergo warranted his credits - look at the likes of Iron Man and War Pigs. His performance at Paris 1970 is still the best live drumming performance I've ever seen, he was going for it like a madman.