>> But the plural of octopus is not octopi — it’s octopodes. <<
No less than “Ask Oxford” agrees with you, but the Oxford dictionary is, of course, British, and the dictionary itself (as opposed to the advice-column formatted website) states all three plural forms - octopodes, octopi, and octopuses - to be valid.
Further, the argument made by “Ask Oxford” is invalid. “Octopus” does not come to us as a Greek word, but as a biological term for the animal’s genus, which is derived from the Latin word which is based on the Greek word. It is certainly standard to pluralize a Latin name for a genus by changing “us” to “i,” although this is not an absolute rule: the plural of “genus” is “genera.”
Thanks for looking that up. A fascinating re-education.