Like I said in my first reply Tak - it's not a hissing sound it's a buzzing sound - apart from that he's right. Australian have long truncated words and added an E or O sound to the end. it wasn't uncommon, mid last-century, for an expat to comment "I'd like to get back to Aussie for my brother's wedding." But Aussie has always been pronounced Ozzie (even though the 'aus' in 'Australia' is always pronounced 'oss').
It's not much of progression to shorten Aussie to Oz - I'm guessing around the mid to late 50s - as evidence by the birth of Oz Magazine
The fact that it references a logic-denying, make-believe land (west of California) run by an incompetent wizard adds to the resonanance.