In the English speaking language the O in opossum is silent and not pronounced and it's just Possum is how we say it in the USA and in English.
In other countries the O might be pronounced when saying possum and they might say it as opossum.
I've always known it as possum and it's how we were taught to spell and pronounce possum in school.
So you just say it as possum and not opossum.
I'm not even sure why there is an o in opossum because possum sounds much better than opossum but it can be said in a few different ways.