Generally... you don't as the consecutive discrete identical vowel sounds doesn't really exist in standard English phonology. However, if you want to express something that isn't formally romanised/anglicised, whether it's fictional or to transcribed from another language with different phonology, it's probably easiest to use an apostrophe. In the case of the repeated short 'o' sound, then "o'o". Or something along those lines (apostrophe instead of a hyphen to indicate it's the continuation of a single word and not two separate words tacked onto each other).