  We study open books (or open book decompositions) of a closed oriented
3-manifold which support overtwisted contact structures. We focus on a simple
closed curve along which one can perform Stallings twist, called ``twisting
loop''. We show that the existence of a twisting loop on the fiber surface of
an open book is equivalent up to positive stabilization to the existence of an
overtwisted disk in the contact manifold supported by the open book. We also
show a criterion for overtwistedness using a certain arc properly embedded in
the fiber surface, which is an extension of Goodman's one.
