Where did the Autralian or British expression good'o come from? What is the 'o part related to?
The OED has a long note on the ‘-o’ suffix. ‘Good-o’ seems to have derived from the practice of attaching ‘-o’ to other words to form conventional cries and refrains, a practice which is attested from late Middle English. ‘Good-o’, however, makes its apperance only in 1916.