Originally Posted by JohnDThompson

Caveat: while that is an inexpensive and effective lockring tool, it really only works on lockrings with an even number of notches. Most of those headset lockrings have only three notches, and the pliers can't effectively grip them. So, the hook spanner is the tool to use in that case. Hook spanners designed for bottom bracket lockrings have too wide a radius to engage the headset lockring, so you either need a smaller radius hook spanner, like the Hozan C205, or one with a hinged hook.