I actually had no idea that project existed… and probably wouldn’t have made this one if I had.
However upon comparison, I would say that oranc is probably more flexible to be used locally or to any OCI registry, while nixcache-oci is more opinionated to a github-based workflow – github builders and ghcr, auto-discovery, scheduled updates and garbage collection, all running from github.
I think that with nixcache-oci I handle narinfo lookups better, using an in-memory index to decrease the number of necessary fetches, while oranc does a more straight-forward per-path OCI fetch (too many fetches could cause problems with rate limiting).
But all in all, they serve the same purpose. oranc is much more mature, of course.