Recently over on the social media hellsite, I offered the following puzzle:
What do the following words have in common? SICK, WINE, RANDOM, WEIRD?
The answer, which a couple people got, is that they all are used to form negative epithets ending in -o. This morpheme is actually somewhat productive: pinko, weirdo, wino, dumbo, sicko, wacko, lesbo, fatso, rando, lameo, maybe also psycho, pedo, and narco if you don’t analyze them as abbreviations.
There are of course a bunch of other words formed using -o as a suffix that aren’t insulting nouns: ammo, camo, repo, demo, aggro, combo, promo, etc. Again, some of these are analyzable as shortenings but others, like ammo for ammunition, have something else going on. But these are different insofar as the role of the -o is not to create a noun describing a person.
Having looked around a while, I can’t find a single one of these epithets ending in -o that’s positive or even neutral. You can’t describe a smart person as smarto or a fun person as a funno (I think?).
The Google Ngram chart for these forms shows them to be largely a late 20th-century phenomenon; wino is the earliest and most popular through the early 90s, now overtaken by far by weirdo, but most of these words seem to emerge in the 1980s or later:
The OED and other major dictionaries don’t identify these distinctly as creating insults; the OED does have an entry for -o, suffix but doesn’t really distinguish these senses in terms of their negative sense or treat them as a class – rather, it distinguishes those that derive from adjectives (weirdo) from those that derive from nouns (wino) which is valid but doesn’t capture what I really think is going on here. Also, many of the -o forms were earlier -ie / -y nouns: dummy, weirdie, fatty.
I think little mini-word classes like these are interesting in that they show linguistic change and productivity on a small scale and in a way that doesn’t really show up in reference grammars and dictionaries. They’re a little aesthetically rich fragment of English informal speech that really, all languages have, but don’t get well-captured in some kinds of formal analysis. And as a language weirdo – or wordo? – I think that’s pretty cool.
Glossographia 
The -o in words like ammo and combo interests me. Doesn’t French do something similar? Dico and so on.
Yes, absolutely, -o is a productive morpheme used to shorten words French and, I’m sure, in other languages too. I don’t see any evidence the English ones come from French or anywhere outside of English itself. I don’t know if other languages are using -o productively to create insults/epithets.
I wonder about Australian hypocoristics? Wikipedia calls them diminutives which I don’t think is really an accurate term, but at least they have a list: https://en.wikipedia.org/wiki/Diminutives_in_Australian_English
Some of the ones that describe people and that are positive or neutral would be: ambo (ambulance paramedic), garbo (garbage collector), journo (journalist), muso (musician), preggo (pregnant), relo (relative) and veggo (vegetarian).
I was going to write a whole additional section on those, but you’re quite right that in Australian English it’s more open to various senses, including positive ones.