On the new OPI setup there’s no star fangled nut, the preload is achieved by tightening the steerer, which is a tube inside the head tube which threads into the bottom of the OPI stem and is braced against the bottom of the head tube.
If you have this setup, you can just loosen the two lefty clamp bolts, then tighten up the steerer – you can see the end of the steerer tube at the very bottom of the head tube (uses a standard Shimano HT2 bottom bracket spanner fitting)If there’s still play after this, then I would assume the bearings need replacing.
Here’s what the stem and steerer look like without the headtube – might help to understand…