Right now I'm using an anecdote about the Japanese Akita, Hachikō. The dog's name contains an "おう" (ou) in Japanese. What is the standard way to romanize this? Should I use a macron (ō) or "ou"? The same goes for the president of Japan, Shinzō Abe.
Honestly, it depends.
The most widely used system of romanization is arguably Hepburn romanization. To quote, "The Revised Hepburn system of romanization uses a macron to indicate some long vowels... For example, the name じゅんいちろう, is written with the kana characters ju-n-i-chi-ro-u, and romanized as Jun'ichirō in Revised Hepburn." (https://en.wikipedia.org/wiki/Romanization_of_Japanese#Modern_systems)
However, other widely accepted romanization methods besides Hepburn, like Nihon-shiki, preserve the exact characters for the word, meaning that you would instead use ou. I've seen it done both ways, and I don't believe either way is wrong.