  Cluster structure of 16O,18O and 20O is investigated by the antisymmettrized
molecular dynamics (AMD) plus generator coordinate method (GCM). We have found
the K^{\pi}=0$_2^+$ and 0$_1^-$ rotational bands of 18O that have the prominent
14C+\alpha cluster structure. Clustering systematics becomes richer in 20O. We
suggest the K^{\pi}=0$_2^+$ band that is the mixture of the 12C+\alpha+4n and
14C+6He cluster structures, and the K^{\pi}=0$_1^-$ band that has the 14C+6He
cluster structure. The K^{\pi}=0$_3^+$ and 0$_2^-$ bands that have the
prominent 16C+\alpha cluster structure are also found.
