Your comment is astonishing.
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.
Because they're a native speaker. Native speakers are often utterly oblivious to the 'rules' of their own languages.
Every time I read a rule about my mother tongue (Mandarin) online I was like, lol what nonsense foreigners made up... And then I realize that rule does exist. I just have internalized it for so long.
Well, speaking for myself, I internalized how う is pronounced differently in different contexts when I was young, and by now I've almost forgotten there's a difference I need to be conscious of.
When I hear /ho:/ in a certain context, "ほう(方)" immediately comes to mind, without noticing that what I heard was a long o. To me it's just the う sound. And if someone pointed to their face while saying /ho:/, I'd think it's the お sound as in "ほお(頬)".