  In $^{16}$O, we investigate the relation between the $E0$ monopole transition
matrix elements and cluster-shell competition using antisymmetrized quasi
cluster model (AQCM), where the dissolution of $alpha$ clusters into quasi
clusters due to the effect of the spin-orbit force is introduced. We focus on
the structure change when the strength of the spin-orbit force is varied. The
ground state dominantly has a compact four $alpha$ structure (doubly magic
structure of the $p$ shell), which is rather independent of the strength.
However the first excited state ($0_2^+$) has $^{12}$C$+alpha$ cluster
structure and this state is largely affected by the increase of the strength;
the subclosure configuration of the $p_{3/2}$ shell for the $^{12}$C cluster
part becomes more important than three $alpha$ configuration. The $E0$
transition from the ground state with compact four $alpha$ configuration to the
$0^+_2$ state with $^{12}$C$+alpha$ cluster configuration is suppressed when
increasing the spin-orbit strength. Although the $E0$ operator itself does not
have the spin dependence, the matrix element is sensitive to the difference of
intrinsic spin structures of the two states. The $E0$ transition characterizes
the persistence of four $alpha$ structure in the $0^+_2$ state.
