  The strength functions of isovector dipole resonances in the even oxygen
isotopes $^{18\sim24}$O are calculated by using an extended version of the
time-dependent Hartree-Fock theory known as the time-dependent density-matrix
theory (TDDM). The results are compared with recent experimental data and also
with a shell-model calculation. It is found that the observed isotope
dependence of low-lying dipole strength is reproduced in TDDM when the strength
of the residual interaction is properly chosen. It is also found that there is
a difference between the TDDM prediction and the shell-model calculation for
$^{24}$O.
