"wo" is definitely o in Hepburn. It is in ISO 3602 too, but that has an exception for wo. I did some research on Japanese wikipedia (四つ仮名) and found that most

"wo" is definitely o in Hepburn. It is in ISO 3602 too, but that has an exception for wo. I did some research on Japanese wikipedia (四つ仮名) and found that most uses of di and du were changed to zi and zu in 1986: ふじ(mountain) and ふぢ (flower) are now both written ふじ. There is a list of words that have changed and ones that haven't. The government decided that the particle wa should be written ha in 1955: 助詞のはは,はと書くことを本則とする (an actual law) I don't really have a point, but I now know a lot about kana usage. ISO 3602 makes a lot of sense to me because it is unambiguous in principal, but also allows phonetic spelling when necessary. With the number of variations of romaji I have seen, even in official usage, it's a wonder why they bother. I have found Japan to be a place that makes a lot of rules, then makes following them optional (then saying "there is no excuse" when you break them and get caught.)