If you know the word 方, that it is /ho:/, and you know that it has a う in it when written out, how can you not know that う stands for making the o long? The only vowel is the long o.
Japanese kindergarten kids can recognize hiragana words with "おう", correctly identifying it as /o:/. By the time they learn the 方 kanji they would have seen it written in hiragana upmpteen times, like AよりBのほうがいい and whatnot.
