The spelling OW is a two-faced letter pair. In 'cow' it says /aʊ/ (like 'ouch'), and in 'low' it says /oʊ/ (like 'oh'). English gives no perfect rule, but strong patterns make most words predictable.
The Rule (and Patterns)
OW has two possible sounds: /aʊ/ (mouth wide open) and /oʊ/ (lips rounded). The most useful pattern: in two-syllable words ending in -ow, it is almost always /oʊ/ (yellow, window, follow, borrow). In one-syllable words, you must learn each, but many short, everyday action words use /aʊ/ (cow, now, how, down, town).
OW = /aʊ/ (like 'ouch')
OW = /oʊ/ (like 'oh')
When OW Changes the Meaning
Some words are spelled the same but change meaning with the sound. 'Bow' is /baʊ/ when you bend forward, but /boʊ/ when it means a ribbon or a weapon. 'Row' is /roʊ/ for a line of things, but /raʊ/ (British) for an argument. 'Sow' is /soʊ/ (to plant) or /saʊ/ (a female pig). Context tells you which.
Practice Tip
Sort a word list into two columns by sound: cow, town, crowd, now versus low, grow, snow, throw. For a related contrast, see our guide to the many sounds of OU.