Here's why:
* The "o" sound in "octopus" is short. It's pronounced like the "o" in "hot" or "cot."
* Long vowel sounds are typically associated with a single vowel followed by a silent "e" at the end of a word, or a vowel digraph (two vowels together). This isn't the case in "octopus."
Therefore, the "o" in "octopus" is a short vowel sound.