In looking for a system URI and OID for the International classification for tumor cancer staging I asked for it on Zulip. The answer is that US Core defines a system URI (http://cancerstaging.org)

It would make sense to add this to the list of FHIR terminology systems for everyone to find and use. Since we do both FHIr and V3, I'd really appreciate an OID for it too.

https://chat.fhir.org/#narrow/stream/179166-implementers/topic/TNM.20Classification.20System.20URI

http://hl7.org/fhir/us/mcode/ValueSet-mcode-tnm-primary-tumor-category-vs.html