I would send a picture as proof but for some reason I can’t?? Anyways, according to my phone’s translation the gravestone does in fact say “oga”! Maybe it’s spelled differently than usual to make it just different enough that Taiyo won’t realize? Or something? Idk. I just know that it is “oga”
Nice catch! I did some digging and the short answer is: you’re absolutely correct, the name on the gravestone is just Oga’s first name written in a different Japanese script.
For a longer explanation (with the obligatory disclaimer that I am not a Japanese language expert, just an obsessive fan with access to search engines):
Oga’s name is normally written in kanji, the most common writing system that the Japanese language uses. In kanji, Oga’s name is written as 王牙. But since he’s supposed to be in disguise, putting his regularly spelled name up there would be too big a giveaway. So to give the costume a level of stealth, Oga’s name is instead written with hiragana (another Japanese script that sort of functions like a phonetic version of kanji). In hiragana, Oga's name is spelled おうが .
Oga's name written as 王牙 (kanji).
Oga's name written as おうが (hiragana).
They both mean the same thing and the kana make the exact same sounds, but if Taiyo had looked over at Oga's disguise and read the inscription, he probably wouldn't have thought "hey, that's Oga's name!" He more likely would have thought "wow, that guy had a really weird surname" lol.
