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Words like tomorrow, sorrow, arrow, follow, borough contain /o/, as in the diphthong /oʊ/, which was /wə(n)/ in Middle English which was weakened from Old English /x/ or /ɣ/ + some sort of vowel. There was no vowel /o/ there in the first place. For example: morrow < ME morwen (no /o/) < OE morgen (no /o/); follow < ME folwen (no /o/) < OE folgian (no /o/).

Has there been a theory on when and why it was inserted there? Also, why does North American English have /oʊ/ consistently regardless of whether the original Old English consonant was /x/ or /ɣ/, while British English has /ə/ for Old English /x/ (as in borough < ME borwe < OE burh) and /əʊ/ (as in bow < ME bowe < OE boga; however arrow in both NAE and BE < ME arwe < OE earh)?

Old English [ɣ] became [w] in Middle English in certain environments when surrounded by voiced segments. Thus /sorɣe/-> /sorwǝ/. This in turn was subject to epenthesis and produced variants like soruwe, meaduwe for Mod Eng "sorrow" "meadow." The final /ǝ/ was subject to deletion like other /ǝ/ in later Middle English. The epenthetic vowel represented by o was retained because a word-final cluster *rw was impossible. The contemporary standard pronunciation with /o/ is at least in part a spelling-pronunciation. In the early 17th century, pronunciations like /windǝ/ and /arǝ/ for "window" and "arrow" were not so stylistically marked, and are described by Ben Jonson as normal pronunciation in reference to o which "in the last Syllabes before w...frequently looseth: as in willow, billow." I.e. /bɪlǝ wɪlǝ/. By the end of the eighteenth century, John Walker was describing it as substandard: “The vulgar shorten this sound and pronounce the o obscurely, and sometimes as if followed by r, as winder and feller; but this is almost too despicable for notice. Good speakers preserve the diphthong in this situation”

If it hadn't been for the 18th century zest for spelling-pronunciations, we might well think of "Sarah" and "Arrow" as being perfect rhymes today. The 18th and early 19th centuries were a period of great social change, social mobility and thus social insecurity in urban areas. The obsession with propriety and gentility led to obsession with propriety in language. At the same time, mass-literacy was spreading on a scale unprecedented in English history. An environment like that tends to encourage language change, and this one in particular was a breeding-ground for new pronunciations derived from spelling. Newly-literate people who spoke various non-standard Englishes, when trying to "better themselves" linguistically, often had no better guide to correct pronunciation than the traditional orthography. And our orthography is a nasty one for learners.