The interwebs have taught me that Oi can often be followed with "you what, mate?!*
Also interesting and not mentioned among the other languages/"see also" sections - "Oyez!" and "¡Oye!"
Oir it's a verb on and by itself and comes from latin audīre and the "¡Oye!" Expression is usually used as a retort, not as an initiator. In cooking settings "¡Oido!" Is used both as a call for attention as an answer.
Only in Britan. In Australia it's usually "oi, dickhead" or - in a call and response format "oi", "what, dickhead?".
Unrelated but “oi, dickhead” is what I shout at my cat when he tries to climb the Christmas tree for the nth time that day.