An object class is identified by a unique object class name, can be described using a class description, can have optional identity ( ID) attributes and optional representative display ( REP) attributes, can be associated with and an optional list of domain-specific properties. and is associated with one or several (member) attributes. Domain-specific properties are specified as tag-value pairs and are meant to be interpreted by application-specific software. An object class can also be associated with update rules (see section 5).
An attribute can be associated directly only with one object class.
However, an attribute associated with object class 
is inherited by
every subclass of 
(see below).
A subset of the attributes associated with an object class
is specified as the identifier for the objects in that class.
Object identifiers are used to distinguish among the objects (instances)
of an object class.
There are two main types of local object classes in OPM: base object classes and specialization (subset) object classes.