It is an O-ring, not ideal, but that's what it is. Considering the slow rotation speed of the steering shaft it's an acceptable solution. The shaft bearing is pretty tight so there is not much leakage when you pull out the O-ring although having the new one at hand is a good idea.
Also make sure the shaft is smooth so you don't tear up the new O-ring sliding it into place. On our upper helm somebody in the past had gripped the shaft with pliers, most likely vise grips, and left some significant burrs.