Dictionary.com Unabridged
oc·to·pus
noun, plural -pus·es, -pi
1. any octopod of the genus Octopus, having a soft, oval body and eight sucker-bearing arms, living mostly at the bottom of the sea.
2. something likened to an octopus, as an organization with many forms of far-reaching influence or control.
[Origin: 1750–60; < NL < Gk oktpous (pl. oktpodes) eight-footed; see octo-, -pod
Dictionaries have all sorts of things these days by illiterate editors and enablers of illiteracy — such as kudo (occasionally the editor will acknowledge that kudos is already singular, but they explain that since so many don’t realize that, they’re going along with the flow.)
Octopi is just an affectation used by people wanting to sound literate. I understand the Greeks used polypous, polypodes.
There is an omega after that tau...
http://itre.cis.upenn.edu/~myl/languagelog/archives/000813.html