Traditionally, words ending in -o tended to refer to people and generally had a negative connotation: wino, psycho, wacko, etc.
However, in Australia it seems that this suffix is being adopted rather widely and it now has a neutral connotation: arrvo (afternoon), avo (avocado), servo (service station), rego (car registration number), garbo (garage collector) and probably many more.
I don't know why some words have -o while others have -ie: barbie (barbeque), hunggie (one hundred dollars), uie (a U-turn) — though I'm not sure how the last two are spelled; I guess they rarely are written, though they are very commonly spoken.
