Using geometric methods and without invoking deep results
from group theory, we prove that a classical unital of even order
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
.
