An informal, shortened way a child may call their father (short for お父さん). Used at home or between close people; sometimes used to refer to one's husband.
お父、あのね、学校でいいことがあったよ。
Dad — guess what, something good happened at school.
お父、今日は早く帰ってくる？
Dad, will you come home early today?
お父は今出かけているから、あとで電話してみるよ。
My father has gone out now, so I'll try calling him later.