When you look up the OID representing FHIR in the HL7 OID registry (http://www.hl7.org/OID/ and search for 2.16.840.1.113883.4.642), it is described like so:

This is the root OID used by the FHIR publication process to assign OIDs to FHIR concepts so they can be re-used in V3 if desired. The list of OIDs assigned by FHIR is available here: http://hl7.org/fhir/oids.htm

The link referenced here does not actually work/exists. Please create this oids.htm page in the spec, or update the link in the OID registry to point to the correct location.