Australian possums and American opossums are marsupial mammals, but otherwise they're nothing alike. Neither the same species nor even the same order. The Aussie possum doesn't even play dead when threatened, the opossum signature move. Nor does it have the silent O.
Captain Cook's botanist Joseph Banks named the Australian critters 'possum' because of their resemblance to the opossum. If he'd named Aussie crocodiles they may well be called gators today.
Having only ever seen it written, I never realised Americans don't pronounce the first "O".