@@Kakaiiwashere Actually, both are correct. Many times when Japanese words are translated to English the 'o' sound is more of a long 'ou/oh' sound, which is written as ou. This can for example be seen in names like 'Shouto Todoroki'. If it doesn't make that sound, but rather a short 'o' sound, like with 'Roronoa Zoro', it's written normally. The reason arigato(u) is both correct is because it was written and spoken differently (due to it being used very frequently, I presume) in different prefectures. TL;DR both are correct, it depends on whether you write the 'o' as an o (short o) or ō (long o)
