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 clust
er 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.
