means dude, guy, or bruvver.
Most of the time it's used when referring to a man not older or the same age as you, because, in the Republic of Zimbabwe like everywhere else, it's rude to address senior citizens like you're talking to your everyday friends. And lest I forget, "oan" is also used by Zim daiges (or "brehettes") when referring to their best friend or a group of chicks (e.g., "oandem").
Blazi or blazo
Mufero, plural mafero
Mufesi or mfesi, plural mafesi
Muchinda or jinda, plural machinda
Note: Oan is pronounced just like the English word "own". It's quite easy, innit? At least that's what I'd think.