"Oi! get over here"
"Oi, I agree"
"Oi, it goes like this..."
The word "oi" is an interjection used to attract attention. It's informal and comparable to American English "hey". Although its origin is unknown, I suspect it came from the words "I say", with "I" having been prounced as [oi] in Cockney.
By they way, "oi" is also an interjection used to attract attention in Japanese. It's informal speech.