What is "Identity" in OOP & ER & SN Data Models?

When trying to map Object Oriented Programming models, to Entity Relationship models, to Semantic Network models, how does the philosophical concept of "identity" get handled? I.E. how is a "thing" identified in each model? [And the following assumes that incorrect criteria is not used e.g. using the "name" of the thing as its "identifier".]

• OOP models assume that the "object pointer" (or object "handle") is a global unique identifier (GUID) for the "thing" represented by that object instance of that class.
  
• E/R models assume that there is either an opaque key (ala sequence numbers) or some set of attributes whose combined values form a GUID for the "thing" represented by that row of that table.
• S/N models assume that there is some explicit or internal key associated with each "entity".