  We report site-selective oxygen isotope ($^{16}$O/$^{18}$O) effect (OIE)
measurements on the in-plane penetration depth $\lambda_{ab} $ in underdoped
Y$_{0.6}$Pr$_{0.4}$Ba$_2$Cu$_3$O$_{7-\delta}$, using the muon-spin rotation
($\mu$SR) technique. A pronounced OIE on the transition temperature $T_c$ as
well as on $\lambda_{ab}^{-2}(0)$ was observed, which mainly arises from the
oxygen sites within the superconducting CuO$_2$ planes (100 % within error
bar). The values of the corresponding relative isotope shifts were found to be
$T_c/T_c= -3.7(4)%$ and $\Delta\lambda^{-2}_{ab}(0)/\lambda^{-2}_{ab}(0)=
-6.2(1.0)%$. Our results imply that in this compound the phonon modes involving
the movement of planar oxygen are dominantly coupled to the electrons.
