The clone of the ballistic type (with the separate NVG mount attached) is a medium sized clone. The clone of the Base Jump type (with the molded NVG mount) is the large/extra large size. I wear a 7 5/8 and I can wear both the real Ops Core Base Jump as well as the knock off of the same helmet.
One thing that helps is that with the OCC Dial system, you can even remove the rear pad in the helmet as it stays on just fine without it, and fit really large heads.
This is the base jump style (clone on top real on bottom):
