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The close-mid back rounded vowel is, according to Wikipedia, "usually diphthongized to [oʊ]".
Examples: row, also.
In fact, in the Cambridge English Pronouncing Dictionary I didn't see o standing by itself.
In some other languages, you can pronounce it just by itself. How common is that and why is English different?
English high and mid non-central vowel phonemes fall into two categories:
Beside the tense/lax phonetic distinction (which refers to the muscles at the root of the tongue), these phonemes are distinguished in several other ways -- language loves redundancy and builds it into the structure whenever possible.
For instance, all the tense vowel phonemes are diphthongized, whereas the lax vowels are pure vowels, which may be neutralized with their tense counterpart in some environments (Mary, merry, marry), but never appear with an offglide except in local variants.
In the high vowels /i, u/ the diphthongization isn't always distinctive because the tongue gesture is so short, but it's there and shows up in transitional /y/s and /w/s in phrases like be able or do it. It's clearer in the mid vowels, where the distance the tongue has to move going from [e] to [i] or [o] to [u] is longer and more easily distinguished.
Indeed, pronunciation of Spanish /e/ as /ei/ and /o/ as /ou/ is one of the characteristics of an English accent. English speakers are normally unable to distinguish the two in Spanish, just as Spanish speakers are normally unable to distinguish English tense vowels from lax, leading to ship sheep late let Paul pole foot boot vocabulary problems.
There are complexities. In RP, as noted, the actual diphthong is centralized to [əʊ], and the Northern Cities Chain Shift has screwed up urban American English vowels almost as thoroughly as what the Great Vowel Shift did to Middle English vowels.
In British English, actually, the diphthongization is [əʊ], while [oʊ] is more US pronunciation.
English doesn't have a single [o] in words, because it's a short vowel and English doesn't have a short "o" sound. The case where it's not followed by [ʊ] is [ɔ:], as in caught.
If you look at this page about English Phonology, and you look at the table reporting the vowels in English, you'll see that the box for "mid back short" is empty.
At first, you misquoted Wikipedia. What it actually says (the current edit) is that "in the north of the Netherlands and in North Central American usually diphthongized to [oʊ]." The same Wikipedia article has lots of examples when it "stands just by itself."
In British English, things are way more complicated. Although [əʊ] is the most common variant now, there are other variants, too; see part 8.10.4 in Gimson's Pronunciation of English (7th ed.).
And there are British dialects where [o:] is not diphthongized. After all, in Middle English it wasn't, either. It got diphthongized during the Great Vowel Shift. There are dialects where the glide is a schwa etc.
For some discussion based on the material of English dialects (the so called goat test), see Wells, J. C. 1981. Accents of English. Cambridge: CUP.
In The Linguistic atlas of England, you can find excellent maps; see, for example, the word "oak" (or loaf, toad etc.), which is not diphthongized in some dialects.
The map of North American English Dialects has vast amount of collected data pertaining to your topic, with a visually appealing presentation.
