Silent letters 

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. Likewise, the phrase should be "a opossum" not "an opossum," again based on the first pronounced letter. 

Therefore, I will go through the Article changing "an opossum" to "a opossum." The O in "opossum" is no different from the H in "honest."