Reasoning with OWL

It depends how you model your facts. When you say "X was seen driving a truck", does it mean that X actually drives a truck? Let me try to model your situation, in Turtle syntax:

:Vehicle  a  owl:Class .          # The class of all vehicles
:Bus  a  owl:Class;               # The class of all buses
    rdfs:subClassOf  :Vehicle .   # Buses are vehicles
:Truck  a  owl:Class;             # The class of all trucks
    rdfs:subClassOf  :Vehicle;    # Trucks are vehicles
    owl:disjointWith  :Bus .      # Trucks are not buses and vice versa
:drives  a  owl:ObjectProperty .  # The relationship between persons and the vehicles they drive
:BusDriver  a  owl:Class;
    rdfs:subClassOf  [
        a  owl:Restriction;
        owl:onProperty  :drives;
        owl:someValuesFrom  :Bus  # A bus driver is someone who drives a bus
    ] .
:X  a  :BusDriver, [              # X is a bus driver
       a  owl:Restriction;
       owl:onProperty  :drives;
       owl:allValuesFrom  :Bus    # X only drives buses
    ] .

Now, if it is asserted that X drives a truck like this:

:X  :drives  [ a  :Truck ] .

then the resulting knowledge base with the ontology is inconsistent and an OWL reasoner can detect this automatically.