Here's a breakdown:
* お (o) is a polite prefix used before words to show respect.
* 父 (to) means "father".
* さん (san) is a polite suffix used after names and titles.
So, お父さん (otōsan) literally means "respected father."
You can also say パパ (papa), which is a more casual term for "father" and is more commonly used by children.