Marking informally shortened or slang nouns.
[Perhaps from the interjection oh!, or the use of -o in ballads to terminate lines; its use has been reinforced by shortened forms ending in the linking vowel -o-.]
Though a wide variety of nouns in English end in -o, this suffix occurs only in words that have been formed from other native words in one of two specific ways. One method is to informally abbreviate a longer term, of which a few examples out of many are ammo, condo, hippo, limo, and photo. Others are based on an adjective or noun, to which the suffix is added to create a colloquial or slangy term, which is often—but by no means always—derogatory: beano (from beanfeast), boyo, cheapo, kiddo, pervo (from pervert), pinko, righto, sicko, weirdo, wino.