Did I see “a opossum” or “an opossum”? It’s pronounced “possum” (at least insofar as my experience) but looks really awkward written “a opossum”.
That is all.
There are two different words. Rather, the same animal name is spelled
opossum əˈpɒsəm BrE; əˈpɑːsəm NAmE
as well as
possum ˈpɒsəm BrE; ˈpɑːsəm NAmE
— both the words are recorded by OALD8.
Each of the words is pronounced according to its initial sound, as usual.
Specifically, one does not write opossum and read it possum. It follows that one writes either “an opossum” or “a possum“.
Grin(ning) like a possum is so often written ‘Grin(ning) like a opossum’ for “scholarly” effect 🙂