If you are writing formally or you are not sure whether your audience will understand which animal you mean, make sure you spell opossum with an O unless you are talking about the marsupial from down under.

Does possum have an O?

The O in "opossum" is spelled but not pronounced. It is a silent letter, just like the H in the word "honest." The articles "a" and "an" are correctly based on the first pronounced letter, not simply the first letter. So, you have the phrase "an honest man" not "a honest man," even though H is a consonant.

Why are opossums spelled with an o?

Possums are native to Australia. When English explorers first met the Australian creature, they thought it looked like the North American Opossum. As a result, they called it a possum and recorded the name like North Americans would pronounce it (without the “O“).