Why is operation pronounced and opossum possum?

You're right, the spelling of "operation" and "opossum" are very similar. However, the pronunciation differences come from how the words came to be in the English language:

* Operation: This word comes from the Latin word "operatio," which means "a working" or "a doing." The "o" in "operation" is pronounced like the "o" in "open," reflecting its Latin root.

* Opossum: This word comes from the Algonquin language, a Native American language group. The "o" in "opossum" is pronounced with a short "o" sound, as in "hot" or "box." This reflects the pronunciation of the word in the Algonquin language.

So, the different pronunciations of the "o" in these words are simply due to the words' origins. They have different etymological roots, which leads to their different pronunciations.