The "o" in "opossum" is not silent. "Possum" refers to a completely different animal.
According to Merriam-Webster, the "o" in opossum is optionally silent.
I'm not sure what to make of the pronunciation of opossum at dict.org, as I can't find a pronunciation guide there (maybe the "*" after the "O" means it's optional? - update: Nope, other examples show clearly it's just a delimiter between syllables).
I find it interesting that only the opossum "plays possum." I think perhaps the "o" is an invisible silent letter in that phrase :-)