According to the Collins and the Cambridge Dictionaries the correct way to say Octopus in Plural is Octopuses or Octopi.
However, according to the following video, we can either use Octopuses, Octopi and Octopodes:



Wrong ways to say Octopus in Plural are Octopii, Octopussies and Octopoids.

The plural of octopus is octopuses. The word octopus origniates from Greek, and the plural form of octopus in Greek is Octopodes. Sometimes you can still see Octopodes are used, it is not stardard but not incorrect.
Source:oxforddictionaries.com/definition/octopus