owl

Hallo,
Given a restriction:
:a rdfs:subClassOf [a owl:Restriction; owl:onProperty :p;
owl:someValuesFrom :b].
should then all members of the class :a have a declaration for the property
like:
:x a :a. :x :p :y.
or can there be members say :x1 that have no such property declaration but
just a declaration:
:x1 a :a.
I'm not sure of this and it has a rather significant impact on the OWL
program I am witing.
If anybody can help me, thanks a lot.