Used as a greeting or to attract someone's attention.
Webster's New World
Oi, an alternative spelling for oy or oy vay, is defined as a Yiddish expression used to express frustration, pain or worry.
An example of oi used as an interjection is in the following sentences: "Oi! I forgot my wallet at home!" which means "Oh no! I forgot my wallet at home!"
(south west England) Eye dialect spelling of I.
Plural form of -os.
Origin of Oi
From the Ancient Greek -οι (-oi), the second-declension masculine nominative plural case ending — the plural form of -ος (-os); compare the equivalents of the Latin second declension (-us → -ī), with which the Grecian case endings are usually substituted in English words, especially in the plural.