I've never had the pleasure of splitting osage, but I don't think it's difficult judging by tales from others. Therefore, I don't think a kerf is necessary. A kerf can be cut carefully with a chainsaw or skill saw, and creates a weak spot for the split to follow. Elm (which I have split), on the other hand, can give it up a little easier by cutting a kerf. Just remember, if you cut a kerf, you will likely violate grain. Try it first without.