First of all, I know next to nothing about Japanese, but this question came up to me recently. I was playing this board game called Shadow Hunters with my friends and on the cards below the English name, the Japanese name of the game was also written: シャドウハンタアス. Well, I noticed that the end of シャドウ was オウ and not オー and I thought that it was kind of wierd. My friend pointed out that maybe it should be read as ou and not ō. Which way is it? Are there rules for this? Thanks in advance.
The syllables written as オオ, オー, and オウ are pronounced the same in Japanese. If you look at the dictionary entry here, you'll see that シャドウ is an alternate form of シャドー. Either way is fine, and the pronunciation doesn't change. Why you'd choose one form over the other, I'm not sure, but I'm guessing in this case it was because the game or packaging designers didn't like the way the 長音 dash looked in the middle of the name. (However, I just looked it up and the name is actually written シャドウハンターズ, so who knows!)