The OED2 has an entry for -o, suffix but as far as dates go, one is just referred to various examples (which are not necessarily the earliest). FWIW, I quote the entry, with the earliest date of citation from each of the examples inserted by me, in brackets.
Forming slang and colloquial nouns, adjectives, and interjections.
The use of the suffix is widespread in English-speaking countries and is especially associated with Australia.
a. Forming interjections, as WHACKO int., WHAMMO int.
b. Forming familiar, informal equivalents of nouns and adjectives, as (from truncated word-forms) AGGRO n., COMBO n. [1926 in sense of a white man who lives with an Australian aboriginal woman, 1929 in usual modern sense], METHO n.1 ; (from complete words) BUCKO n., KIDDO n.; CHEAPO adj., NEATO adj.
c. Forming personal nouns from non-personal nouns, as MILKO n.[1905, earlier as a milkman’s call], WINO n.1 
d. Forming nouns from adjectives, as PINKO n.[1930; 1925 as adj. meaning “leftist:, 1917 as adj. meaning “drunk"], WEIRDO n.