I think the law defines concealed weapons. A knife must have a blade longer than 3.5 inches to even be defined as a weapon by state law. Some cities in Nebraska have stricter limits (3" in Hastings for one)
I think that like open carry of firearms, the law is mute on the issue of open carried knives in our state, so I would like to assume that any act not expressly prohibited by law is legal.
Be careful though that you know the local ordinances too, as there is no statewide preemption concerning concealed weapons, other than a handgun carried by a valid CHP holder