  We investigate excited states of $^{18}$O by using the antisymmetrized
molecular dynamics. It is found that five different types of cluster states
exist which we call ${}^{14}{\rm C}+\alpha$, higher-nodal ${}^{14}{\rm
C}+\alpha$, two molecular states, and $4\alpha$ linear-chain. The calculated
$\alpha$-decay widths are compared with the observed data. The higher-nodal
${}^{14}{\rm C}+\alpha$ cluster states reasonably agree with resonances
reported by the recent experiments. We predict that the $\alpha$-particle
emission is dominant for the ${}^{14}{\rm C}+\alpha$ cluster states while the
molecular states prefer the $^{6}$He emission.
