in ortho nitrophenol, there is intermolecular H bonding and this makes loss of proton difficult, so it is less acidic than para nitrophenol.
Remember Cedric. Remember, if the time should come when you have to make a choice between what is right and what is easy, remember what happened to a boy who was good, and kind, and brave, because he strayed across the path of Lord Voldemort. Remember Cedric Diggory.
Albus Dumbledore
Goblet of Fire, Chapter 37, Page 724