I get this, too. All the hair underneath at my nape is fine, and it knots at the drop of a hat. Oiling helps somewhat, but the best way I've found to deal with it if it's really a mess is to use a Tangle Teezer. It still takes a while to detangle, but it's much easier and less painful. Unfortunately, I've yet to find a way to stop it from knotting up in the first place.