Had to google it. Apparently 2 ways to "O-ring the head". Generally means a groove cut in the head for a stainless-steel wire, which aligns with the softer "fire ring" of the head gasket to crush into it. The wire sits proud of the head surface ~10 mil. Stronger sealing is a "fire ring", which is a similar mild-steel wire in a groove cut into the head, but it squashes against the block. You cut out the integral fire ring in the head gasket to allow that. The later might only be done on Cummins diesel engines with high boost (1st link).
But, raises a thought. Owners of 273 engines have fretted over the over-size fire rings on the "interchangeable" small block gaskets, since that leaves a large quench zone. Perhaps it fills with carbon so no issue. If not, seems you could install a mild-steel fire ring in a groove, which would be inside the gasket's fire ring so you don't need to cut it out, giving a belt & suspenders sealing. Downside is looks like ~$500 per head, plus high shipping unless a local shop does it. Would be great if the Chinese aluminum head makers would offer this, at least for the 273, for say $100 extra. The ring might have to be soft copper for an aluminum head to squash it.
https://www.ruttsmachine.com/assets/pdf/literature/en/Fire-Ring-vs-O-Ring.pdf
Cylinder Head - O.Ring Service