The word "おとうさん" or "Otousan" means "father" in Japanese. The 'o' at the beginning is used as an honorific. 'tou-san' is just as acceptable, however it is much more informally used by a member of the family, or might even be used when a child is talking about their father to another person.

In Kanji, it is written as 父.

When reading/writing this word, it can be pronounced two ways depending on how you use it. "Otousan" is the more formal and appropriate way of saying the word father. Children may address their fathers like this if they wish to be polite. However, it is more common for children to address their fathers as "Chichi"(ちち).