Knife charges usually end up falling under city/local ordinances and those change every few miles.
Open Carry is legal in Ohio via "not being illegal or mentioned in the ORC." This creates the mentioned "grey" area for law enforcement. They can attempt to charge you with a couple different things, but a good lawyer can get the charges dropped unless you did something really stupid.
I conceal and open carry all the time. I mostly open carry if I'm with the wife and kids which reduces my threatening appearance;-) Most of the places I go allow open carry as a corporate policy and the greater columbus area police depts are pretty friendly about it. You run into trouble when you get a hick cop or town like cleveland heights, which is still bent out of shape over their gun ban being removed by the ohio supreme court.