OUGH which comes at the end of the word is the least common spelling for /ō/. This phonogram has multiple sounds but /ō/ is not common and is found in the middle of the word. OU that may not be used at the end of English words. OW that may be used at the end of English words and most commonly found at the end of the word. OA two-letter /ō/ that may not be used at the end of English words. Single-letter o can also say its long sounds before two consonants. Single-letter o in a closed syllable ending with a silent E because the vowel says its long sound because of the E. Single-letter o at the end of a syllable because o usually says /ō/ at the end of a syllable.
There is no specific spelling rule to help determine which spelling to use however, there are some helpful spelling patterns to consider when spelling words with /ō/. As with many English vowels, there are multiple ways to represent /ō/ when spelling words.