Just as an aside:
The romanization “oh” is occasionally seen, right? A notable case I can recall is the usual transliteration of the name of Japanese baseball great Sadahuru Oh. I presume using “oh” for Japanese long “o” is no longer done for common nouns and other lexical words.
How about that affect some folks use when they say the word “no”. I comes out something like “NO-uh”. Drives me batshit.
To answer Amikejo’s question in the OP, however:
Other languages (more precisely, dialects thereof) do use the /oʊ/ diphthong. It’s a small number of languages – that specific diphthong is uncommon. However, the general class of sound to which /oʊ/ belongs is not particularly rare – several examples have been given in this thread ([œʊ̯], [əʊ], [o̞u̯], [ʌʊ]). To most all English speakers, any of those sounds will be heard as the so-called English “long o”.
To untrained English speakers, a protracted “long i” in song – as you rightly point out, often rendered as [aaaaaaaaaaaai:] – is interpreted somewhat differently from a protracted long IPA [o:].
To be more precise: In song, English speaking listeners will seamlessly interpret IPA [ooooooo:] as English a sustained “long o” (IPA [oooooooʊ:]).
On the other hand, the [a] part of [aaaaaaaaai:] is itself an independent vowel in English that doesn’t neatly substitute for IPA [ai]. Some English speakers (incl. yours truly in casual speech among familiars) will pronounce the words “time” and “Tom” similarly. But most English speakers will perceive this as a mark of “another dialect” or, less charitably, as “speaking incorrectly”. An English speaker substituting [o] for [oʊ] doesn’t quite carry the same baggage – though native English speakers often notice the [o] for [oʊ] substitution as part of a generalized “foreign accent”.
As I was thinking about the various examples of the sound in question, I began to wonder if Portuguese has this sound. Portuguese differs from Spanish in that there are dipthongs, while Spanish really sticks with pure vowel sounds.
eu vou embora
eu sou marinheiro
meu vôo esta chegando
The last one is a different sound, more closed, but my gringo ears can’t really tell if either of these is really supposed to be pronounced with “oʊ”
That’s how I pronounce them.
Correct, there are different Romanization systems that might render the exact same long vowel as ou or oh. There is also oo, which is pronounced the same but indeed spelled differently in Japanese syllabics.
The surname Oh is apparently from Chinese and is the equivalent to Wang. And of course the Korean Oh/O is completely different.
And of course another baseball player is Ōtani Shōhei, which gets rendered to Ohtani but there’s no reason why he couldn’t also be Outani except that the H keeps sportscasters accurate.
Wonderful recording — thank you! I love medieval and renaissance chiral music, and this is lovely.
Perhaps the chorus — either on purpose, or accidentally — was properly pronouncing the Latin “o,” which I assume was (as best we can know) a pure mid-vowel, like its descendants in Italian and Spanish?
(“Accidentally” because, as bordelond noted, we often subconsciously change the quality of vowels when we sing.)
It still amazes me that the English so-called “long i” is really (usually) just AHH and then EEE. But it clearly is.
In the intro to the song “Crazy Train,” Ozzy Osbourne sings this diphthong-or-vowel-sequence, but with the two parts on very different notes (an interval of a fifth, I think).
So, some websites interpret this lyric as the English word “I,” others as an exclamatory “ay” or “ayy.”
“Oh” has never been used for common vowels in most (all?) romanization systems; names seem to be the exception.
The most common romanization system, modified or revised Hepburn, uses a macrons across a, e, o, and u (like this) ō to make them long, while the two other major systems, Nihonshiki and Kunreishiki, have this symbol, ô. Hepburn (and I presume the others) repeats the i (ii) for the long i.
For names:
This is close but not quite correct. See below.
No. This is incorrect. Ōtani’s name would not be written as Outani. It’s Ōtani, Ohtani or Ootani, but not Outani.
The romanization of Japanese follows the Japanese “spelling” of words. Some Japanese kanji are おお (oo) and others are おう (ou). If oo and ou are used, they must follow the Japanese spelling. Ōtani in Japanese is 大谷 (おおたに) (large valley) so a “non-Hepburn rōmaji” must be as outlined in the previous paragraph.
Wikipedia says, “At least in European Portuguese, the diphthongs [ɛj, aj, ɐj, ɔj, oj, uj, iw, ew, ɛw, aw] tend to have more central second elements [ɛɪ̯, aɪ̯, ʌɪ̯, ɔɪ̯, oɪ̯, uɪ̯, iʊ̯, eʊ̯, ɛʊ̯, aʊ̯]”, it also says “/ow/… [m]erges optionally with /o/ in most of modern Portuguese dialects, excluding some regions in northern Portugal.” But maybe in Brazil they throw an oʊ̯ in there?
It could be either English sound that intrigued you (in how English speakers mispronounce the first vowel in Spanish word “burrito”):
Ah you got it, I did not check the kanji. Usually long o is ou, but 大 is indeed one of those that goes oo
Cuban-American comedian Jenny Lorenzo just released a video that helps demarcate the difference between IPA /o/ and /oʊ/. Forward to about 1:10 when Abuela is asking her Americanized granddaughter Laritza to say her name. The relevant section of the video is about 1:10 - 1:44.
Laritza: Laritza Diaz-Coto (“Coto” with the American English /oʊ/ - b)
Abuela: (scoffs and mocks) Laritza Diaz-/kʰoʊʊ’tooʊʊʊ/! Es Laritza Diaz-/ko’to/!
(Yes, Lorenzo is playing both roles 
)
Our niece had English-Speaking Mrs Jones /oʊ/ & Welsh-speaking Mrs Jones /o/. She couldn’t understand why people would ask her “which Mrs Jones?” since to her the names sounded quite distinct, and she didn’t realize they were the same word in two different accents. (Welsh spells /oʊ/ as [ow], and /o/ as [o].)
Are there double-O words in Afrikaans like in Dutch (e.g. “noot”)? Is that a /œʊ̯/ sound (In either Dutch or Afrikaans)?
too late for edit:
I meant “oʊ”, not œʊ̯.
There are -oo- words, like noot or boot (boat). In Afrikaans, though, those are a //ʊə// sound (a bit of an oo-eh diphthong), not like Dutch at all, where it is the //oʊ//
Learning the difference in vowel sounds between words that are otherewise identical in spelling and meaning is possibly the biggest challenge in learning Dutch for an Afrikaans speaker.