When you talk to your father, you use 'お父さん' to say "you" to him.
When you talk to someone else about your dad, you say "ちち" to mention him.
When you talk about someone else's father you say お父さん to mention him.
When you talk to someone who's a dad (because he's with his kid, for example), you call him お父さん.
That's it!
P!