  Oxygen isotope substitution is performed in the perovskite cobalt oxide
(Pr$_{0.7}$Sm$_{0.3}$)$_{0.7}$Ca$_{0.3}$CoO${_3}$ which shows a sharp spin
state transition from the intermediate spin (IS) state to the low spin (LS)
state at a certain temperature. The transition temperature of the spin state
up-shifts with the substitution of $^{16}O$ by $^{18}$O from the resistivity
and magnetic susceptibility measurements. The up-shift value is 6.8 K and an
oxygen isotope exponent ($\alpha_S$) is about -0.8. The large oxygen isotope
effect indicates strong electron-phonon coupling in this material. The
substitution of $^{16}$O by $^{18}$O leads to a decrease in the frequency of
phonon and an increase in the effective mass of electron ($m$$^\ast$), so that
the bandwidth W is decreased and the energy difference between the different
spin states is increased. This is the reason why the $T_s$ is shifted to high
temperature with oxygen isotopic exchange.
