Affectionate, childlike term for one's father, similar to お父さん but more casual and intimate. Often used by young children or within close families.
See also: お父さん (おとうさん)
Daddy, take me to the park today!
My dad is good at cooking.
Formed by adding the affectionate suffix ちゃん to お父 (oto), a prefix for father. The reading おとうちゃん is a euphonic change from おとちゃん.