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 :-)
In reply to Re^2: My favorite silent English letter is:
by runrig
in thread My favorite silent English letter is:
by chacham
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