Short /ɒ/, long /əʊ/, and a stubborn group where o says /ʌ/ — son, come, love — for reasons that are about handwriting, not sound.
Letter sounds
o LETTER_O_01
Useful pattern · Medium reliability · Very common · Early · Reading
/ɒ/ short o: hot, dog, top
/əʊ/ long o: home, go, old
/ʌ/ u sound: son, come, love
/uː/ oo sound: do, to, who
/ə/ schwa: lemon, carrot, wagon
Where Wiggler teaches it: Skill 2: CVC Word Mastery · Skill 6: Magic e · Skill 12: Multisyllable Decoding
Child explanation
O says /ɒ/ in hot and its name in go. In some words — son, come, love, money — it says /ʌ/ instead, and those just have to be learned.
The rule
Short /ɒ/ in a closed syllable, long /əʊ/ in an open one and before -ld and -st. A closed group of common words has o for /ʌ/, and they are learned as a family.
What is really going on
Short and long o behave according to syllable type with good consistency. The /ʌ/ set is a genuine irregularity with a known cause: in Middle English manuscripts the letter u was written as a sequence of vertical strokes that became illegible next to m, n, v and w, so scribes substituted o. The spelling records the scribe's convenience and the pronunciation records the original u.
Why English does this
Son, come, love, money, honey, wonder and month all had u in Old English. The o is a thirteenth-century legibility fix that outlived the handwriting problem it solved.
Position in the word
Middle of a word — Short in a closed syllable. For example: hot, dog, stop, lock.
End of a word — Long when it ends an open syllable. For example: go, no, so, open, robot.
Before a consonant — Long before -ld, -lt, -st: the old/most family. For example: old, cold, colt, most, post.
Unstressed syllable — Schwa. For example: lemon, carrot, wagon, button.
Examples
Easy examples
hot, dog, go, home, old
More examples
top, stop, lock, box, no, so, open, cold, most, post, bone, rope, note
Made-up words that follow the rule. A child who reads these has the pattern rather than the vocabulary.
blot, shope, grold
Exceptions
o saying /ʌ/: son, won, ton, come, some, done, none, love, glove, above, money, honey, month, front, wonder, other, mother, brother, nothing. A medieval handwriting substitution, not a sound rule. A closed family worth naming.
o saying /uː/: do, to, who, into, movie, prove, lose. A tiny group of very high-frequency function words.
o before ss, ft, ng: cross, off, long, song. In some accents these take /ɔː/ rather than /ɒ/ — cross, off, long, dog.
Contrast pairs
not versus note — Closed versus silent e.
cot versus cold — Closed versus the -old family.
sot versus son — Expected short o versus the /ʌ/ family.
Common reading mistakes
come: a child reads /kəʊm/, where the word is /kʌm/. Magic e applied where the word is irregular.
most: a child reads /mɒst/, where the word is /məʊst/. Short o applied to an -ost family word.
Teacher note
The o-says-/ʌ/ group is large, frequent and arbitrary, and it is worth naming as a set rather than meeting it one surprise at a time. Tell children the real reason — a scribe could not read his own handwriting next to m and n — because it is true, it is memorable, and it tells them the word is not their mistake.
Accent notes
General American — Short o is /ɑː/ — cot and caught may be identical for many speakers, and hot rhymes with the first vowel of father.
British English — Short o is a distinctly rounded /ɒ/, and cot and caught are separate words.