How come some people say “Namu Amida Butsu” and some say “Namo Amida Butsu?” What is the difference? And when and why did “namo” start to be used?
The simple answer is it makes no difference. What is important is forming the idea and then saying the name (myogo) of Amida Buddha.
The six Japanese characters that make up Namo, or Namu, Amida Butsu are a transliteration, or the approximate sound, of the Sanskrit into Japanese. And Amida Buddha’s name spoken in other languages, of course, would be pronounced differently.
But it is the meaning that is important, not the way one pronounces the words.
There is also a widely repeated response to this question that claims that the Nishi Hongwanji is supposed to say “Namo” and the Higashi Hongwanji is supposed to say “Namu.”
However, this is not a satisfactory answer. On a strict doctrinal sense, the scholars tell us the “correct” pronunciation is “NAMO” and this is according to the notes left by Shinran.