O Totoro is the biggest of the three totoros. He's jovial and speaks with a very low, bellowing voice.

Mei meets him after following Chibi Totoro and Chu Totoro through the yard and into the woods. Satsuki meets O Totoro later, while waiting for her father at the bus stop in the rain.

In the English version, Setsuki and Mei's father refers to him as the "King of the Forest."