A battlecry, often used out of content of a battle to signify great excitment about a subject, usually used with the subject before the cry itself, originally from "Thundercats". The number of 'O's is up to you.
Used to show extreme passion and expression towards anyone or anything. Usually follows an exciting event or victory. An experienced user of the phrase may say it at random out of pure addiction to the word.
A Mexican guy came into my yard and wanted to use my phone. The only words he knew in English were "No police" and "E.T. phone home" He was probably an illegal immigrant. True Story.