Adding the letter O onto the end of root words like - friendo, fatso, f*cko, weirdo, etc

Michael Quinion, Ologies and Isms: Word Beginning and Endings has an interesting suggestion regarding the origin of -o as an attachment to freestanding nouns or adjectives:

-o 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 terms, which is often but by no means always derogatory: beano (from beanfeast), boyo, cheapo, pervo (from pervert), pinko, righto, sicko, weirdo, wino.

The attachment of -o endings in ballads goes back centuries, as for example in the ballad "The Bonnie Lass o' Fyvie," which in one version attaches -o to no fewer than sixteen normal English words, as well as to the proper names Fyvie and Peggy. All of these -o words fall at the ends of lines (in fact at the end of the second and fourth lines of each of the ballad's quatrains) corroborating the notion that -o plays a line-terminating role in at least some ballads. Nevertheless, citing that use of -o endings as the source of current -o words, even if it is correct (which is not beyond question), doesn't explain why people chose -o for that purpose way back then. It basically just kicks the can a few centuries down the road.

As Quinion observes, wordos are inherently slangy and often have negative connotations. But in the examples that Quinion lists, some -o words aren't negative, and some that are negative are informal alterations of words that are themselves negative (cheap, sick, weird—to which we can add dumb, creep, lame, stink, and so on). To my mind, the crucial function of -o in modern usage isn't to render a neutral word negative; it's to slangify and informalize a staid, sober, reputable word—often in tandem with converting the word from an adjective into a noun.

Slang Words Ending in “O” Daily Writings

These have been around for a long time. Some actually make it into the registries: condo & cheerio. In slang use they call attention to the base meaning of the word: f**ko doggo beardo and the list goes on. One frequent U. S. use is 'wrongo -dude!'

Interesting example:

There used to be a famous cigarette lighter named “Zippo”. There was also a Japanese bomber code named “Betty”. Betty would catch fire easily when hit by allied fire. The Yanks took to calling them zippos, and even the Japanese crewmen called these planes the same.

And more from the Aussies: Aussie Slang

The OED attributes use of this slang -o suffix to three factors.

the final syllable of words of chiefly Romance origin

Think bongo, mango, and calico.

the vowel that became final after the shortening of a word by dropping the syllables following a medial o , especially in compounds truncated after a prefix or combining form ending in -o

In this case, think of slang words cut short after a medial o, like using the word psycho instead of psychopath.

ho int.1, O int., and oh int., occurring as a second element in various exclamatory phrases. The suffix is attached both to full words and to truncated forms of words and phrases.

Here the OED is referring to derivatives of words followed by Ho! like Righto, which derives from Right ho.