No, and yes. The common name of the Virginia opossum, the only marsupial species native to North America, is usually pronounced “oh-possum” when spelled with the initial “o.” However, many people shorten the word to “possum” for convenience, much as we abbreviate words like “you are” to “you’re.” Spelling it “opossum” but not pronouncing the “o” is also an accepted, official pronunciation.
In scientific or formal speech and writing, the full “opossum” is the preferred pronunciation. In everyday conversation, it is acceptable to treat the “o” as a silent letter if you so choose. According to the Merriam-Webster dictionary, in etymology, the study of words and languages, this natural “loss of a short unaccented vowel at the beginning of a word” is called aphesis.
In Australia and the surrounding islands, the native marsupial possums are simply “possums.” The word does not contain the letter “o” at the beginning – usually. In rare instances, the Australian possum is also correctly referred to as an “opossum.”