I’ve seen it spelled as o’possum and thought the practice arose from seeing it as ‘possum when the “o” is left off. It’s [from] a native North American language word.
...The word opossum comes from the Proto-Algonquian aposoum, pronounced *wa˙p- aʔθemw, meaning “white dog” or “white beast/animal”. ...
Interestingly, there are some special terms listed here:
male ~~ jack
female ~~ jill
baby ~~ joey
group ~~ passel
voice (call) ~~ hiss; growl; purr
It seems as if these terms are for all marsupials. The words sound Australian to my tender Leftpondian ear.