Consider this scenario: You wake up in your condo and pop a couple of Eggo waffles in the toaster-oven. You scroll through your phone, looking for info about a preso you're supposed to sit in on. There it is – along with a photo and a heads-up about a product demo. Memo to self, you think: Must schedule convo about a resto reso for a baby shower. Yes, Margo's preggo!
Hey, presto: You've used thirteen words ending in -o, including one brand name, nine truncated nouns, one colloquialism, one word borrowed from Italian, and one pet name. And it's only 7 a.m.
English loves its o-ending words with a curious fervor, considering how seldom they occur naturally in our mother tongue. (Some exceptions: the verb go, from Old English; fro, a Middle English word meaning "from" that's now encountered mostly in the quaint phrase "to and fro"; and the interjection hello or hallo, which we've had since the 16th century.) For centuries, we've made up for that lack by importing or coining words that end in o. And we've used an ingenious array of tricks to do so.
Latin words borrowed from religion, philosophy, or medicine were the earliest o-suffixed imports. Old English had credo (a belief or creed); Middle English had ergo (therefore) and, thanks to the Black Death, bubo (an inflamed swelling).
War and trade introduced English-speakers to new -o words from Spanish, Italian, and Portuguese: presto (from 1598 on), manifesto (1620), politico (1630), cargo (1657), and a sheaf of musical and theatrical words, including soprano, contralto, scenario, concerto, and pizzicato (all from the 1700s). So infatuated were English-speakers with the -o ending that they sometimes changed spellings and pronunciations to create it: That's how Portuguese manga (itself probably a borrowing from Malayalam) became mango in the late 16th century.
The excitement around these exotic imports may have precipitated a new word-making trend: clipping off the ends of longer words to create short words ending in o. The earliest of these may have been plenipo, which was used by the poet John Dryden in 1691 as an abbreviation of plenipotentiary; Dryden may just have wanted to force a rhyme with "show," but the new form proved popular, especially among the writerly set: Richard Steele, Henry Fielding, and Lord Byron all used it. Then along came memo (first print appearance: 1705) as a shortened form of memorandum, which had been used in English since the late 14th century; and hypo, a now-obsolete abbreviation of hypochondria that was all the rage in the 1700s and 1800s.
By then, the -o suffix fad, the OED tells us, "seems to have become assimilated into English phonology," particularly in imitation of Romance words such as stingo, slang for strong brandy, and rhino, which was slang for "money" 150 years before it was an abbreviation of "rhinoceros." The trend gathered steam in the 19th century, as more words were shortened after a medial o, including Anglo, mezzo, and typo. (Typo was originally a clipping of typographer; it began to mean "typographical error" toward the end of the century.) Queen Victoria, of all people, is the first person to have used photo for photograph (in an 1860 letter). Such abbreviations, according to the OED, "probably established an association of the ending -o with casual or light-hearted use which it has retained ever since." It's certainly true about personal nicknames like Margo (from Margaret) and Bo (from Robert).
Still other o-suffixed words came about by adding the interjection ho or oh to an existing word. Heave ho! is an old sailing term, used since at least the 1400s; cheero, a nautical greeting from the early 19th century, evolved into cheerio a century later – a farewell expression that quickly became a British cultural stereotype. Bottle-oh began appearing in the late 1800s as a cry used by buyers and sellers of bottles; kiddo also dates from that period.
Beginning in the early 20th century, new o-suffixed words began proliferating, and the trend has never abated. When complete nouns and adjectives gained an o, their meanings turned derogatory: wino, sicko, cheapo, weirdo. When words were truncated after an o or other vowel they sounded chirpy or familiar: info (from information, 1907), ammo (from ammunition, 1911), combo (originally a jazz term, from combination; 1920s), porno (from pornography, early 1950s), limo (from limousine, 1960s), condo (from condominium, 1960s), oppo (from opposition, circa 1990). Medical terms like amniocentesis and poliomyelitis have yielded without loss of dignity to their shortened forms. Meanwhile, Australian English elevates -o abbreviations to a popular art form: arvo (afternoon), aggro (aggressive), ambo (ambulance), avo (avocado), muso (musician), povvo (poverty), preggo (pregnant), and many more.
Corporate-speak has been another productive source of -o words. A preso is a presentation, a convo is a conversation, a demo is a demonstration (unless it's a demographic segment). You might ask to be added to the distro (distribution) of a memo from the devo (development) team, or receive a reco (recommendation) for a resto (restaurant). You can even make a reso, or reservation, at a resto.
And speaking of the language of commerce, there's a long and successful history of brand names that end in o, from Beano (a flatulence preventative) and Brillo (scouring pads) to Detecto (medical scales) and Zippo (lighters). All of these brands have roots in the 20th century – Detecto was founded in 1900 and is still thriving – but a few recent tech (techno?) brands follow the same pattern. Carto (from cartography) "turns location data into business outcomes"; Twilio (a fanciful name meant to suggest "telecommunications"), a "cloud communications platform as a service." (I don't quite know what those things mean; I'm just the messenger.)
Then there's Rovio, the Finnish mobile-games company that makes Angry Birds. The name has no connection to English rove, I was told by Finnish speaker HugoVK, although the association probably doesn't hurt. Instead, the name is derived from the Finnish word polttorovio, which means "pyre." It seems that even in Finnish, the urge to create new o-suffixed words is irrepressible.