Using geometric methods and without invoking deep results from group theory,
we prove that a classical unital of even order $n\geq4$ is characterized by two
conditions (I) and (II): (I) is the absence of O'Nan configurations of four
distinct lines intersecting in exactly six distinct points; (II) is a notion of
parallelism. This was previously proven by Wilbrink (1983), where the proof
depends on the classification of finite groups with a split BN-pair of rank 1