Oz has a very mournful-sounding cry that he makes when he's unhappy for some reason, like because he's hungry or J's gone to work or I'm too busy making dinner to pick him up. It's a very sad sound and it makes you want to go and cuddle him. He also has an excited "chirrup" chattering sound that he only makes when he's in hunting mode; you can tell when the Fat Squirrel of Doom is in the window, because Oz's tail will start lashing back and forth and he starts making that chattering noise. (The Fat Squirrel of Doom is a very fat, very cheeky black squirrel who likes to sit on the window ledge in our living room and waggle his tail at the cats. He knows they can't get him and is very cocky as a result. Everyone has an archnemesis; the Fat Squirrel of Doom is Oz's.)
Spike, on the other hand ... Spike makes this sort of bleating noise, like a demented goat: "Meh! Meheheh!" Sometimes he does it because he wants something (usually dinner); sometimes he does it because he's startled or he thinks you might step on his tail (because his tail and your foot are in the same room together, which for some reason in his mind equals immediate tail-stomping ... I don't know why, because to the best of my knowledge his tail has never been stepped on); sometimes I think he just does it because he's bored and likes the sound of his own freakish voice. He doesn't make any other sound, but he makes this sound count: he is very, very loud when he bleats at us.
I know of cats that do, in fact, say "meow," but my cats are not those cats.