Chohinun is a humble word for Oori.
There're two ways to show your respect to someone. One is to ennoble someone or his/her action or anything related to him/her. The other one is to make yourself humble.
You say "Oori" to me, you and I will be on the same stair. But if you say "Chohi", you're on the stair lower than the stair where I stand on. I look down at you, you look up at me and there's some distance between us. So you don't need to use it when talk to your friend.
Cho is a humble word for Na, I'm sure you can figure out when to use it ;)