Japanese people call mother (o ga sang ) why sometimes directly called (ma ma )

This way, ma ma is written in Japanese as ママ, and the other is お母(かあ)さん.

The former is a foreign word, the latter is Japanese.

Very young children (until the end of kindergarten) can be called mama. so, the sub mama also means something like pet. That's why you can sometimes hear older children or adult children, or even older children, call them mama.

O mother (かあ) san, compared to mama is more formal and respectful.