An ocker is the archetype of the traditional working class Australian male.
Paul Hogan, of Crocodile Dundee fame typified the late twentieth century ocker, with his thick Aussie strine (the local lingo), his uber-casual manner of dress (and that's being polite!), and his traditional skills ("ya can fix anything with a reel of baling wire, mate).
The uglier side of the genuine ocker is a propensity for bad language and loutish behaviour. Few Australian urbanites would be proud to be called ockers, but quite a few average Aussie blokes wouldn't mind too much, so long as you weren't taking the piss (having a joke at his expense).