An object identifier (OID) is an extensively used identification mechanism jointly developed by ITU-T and ISO/IEC for naming any type of object, concept or "thing" with a globally unambiguous and persistent name. It is not intended to be used for transient naming. OIDs, once allocated, should not be re-used for a different object/thing.

It is based on a hierarchical name structure based on the "OID tree". This naming structure uses a sequence of names, of which the first name identifies a top-level "node" in the OID tree, and the next provides further identification of arcs leading to sub-nodes beneath the top-level, and so on to any depth.

A critical feature of this identification mechanism is that it makes OIDs available to a great many organizations and specifications for their own use (including countries, ITU-T Recommendations, ISO and IEC International Standards, specifications from national, regional or international organizations, etc.).