  We have prepared oxygen isotope exchanged crystals of impurity-free YBCO with
various oxygen concentents, and examined pure doping ($p$) dependance of
isotope effect on superconducting transition temperature. With decreasing
oxygen contents, the isotope exponent $\alpha$ monotonously increases without
any anomaly around $p = 1/8$. The monotonous increase in $\alpha$ indicates
that phonons are involved in the mechanism which causes the monotonous $T_c$
suppression with underdoping.
