The suffix –o has been long used in English-speaking countries to form slang and colloquial words. Weirdo, aggro and cheapo date back only to the 1960s, but kiddo is older – it was around in the late 19th century. The –o often replaced an earlier –y or –ie, a diminutive suffix; weirdie, cheapie and kiddy were around before weirdo, cheapo and kiddo.
It was in the late 17th century that words containing a medial o, particularly where the o was the final letter of a prefix or combining form, began to be truncated to a form ending in –o: memo, for instance, short for memorandum, and hypo, a word in common use in the 18th and 19th centuries, short for hypochondria. Loco began to be used as an abbreviation of locomotive in the early 19th century. From this practice, the ending –o came to be associated with informal words, used casually or light-heartedly.
Tagging –o, -oh, or –ho on to words as a worker’s cry or interjection, such as heave-ho and hey-ho, is attested from the early 15th century. Alive-oh, shouted by fish-sellers, dates back to the early 18th century. In the 19th century it became more common to tag –oh or –o on to any word when shouting out what goods you had to sell or when making a public announcement: spell-oh was a call to workers saying that it was time to rest, and smoke-oh (chiefly Australian and New Zealand English or nautical slang) told workers it was time to take a break to have a smoke. The OED notes that the –o ending is particularly associated with Australian and New Zealand English, and gives the examples rabbit-o (the call of someone selling rabbits) and bottle-oh (the call of someone selling or collecting bottles). The earliest example of a word of this type being written as a single word (1860s) is milko, the call of a milkman.
From the late 19th century/early 20th century the tagging on of an –o to a word became very common: beano is one of the earliest (originally short for bean-feast), wino, whacko, whizzo, pinko. So fashionable were such words at this time that manufacturers named their products this way, too: Oxo (1899), Bisto (1910), Jell-o (1897).