  We report a study of oxygen isotope effects on low temperature specific heat,
magnetization, and resistivity of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and
(La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$. For the metallic compositions
of La$_{1-x}$Ca$_{x}$MnO$_{3}$ and for charge-ordered
La$_{0.5}$Ca$_{0.5}$MnO$_{3}$ no change in low temperature specific heat has
been detected with $^{16}$O -$^{18}$O exchange, while compounds of
(La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ (0.4<y<0.6) show a significant
change in low temperature properties. The low temperature specific heat
indicates a presence of the charge-ordered phase even in compositions of
(La$_{1-y}$Pr$_{y})_{0.67}$Ca$_{0.33}$MnO$_{3}$ which are metallic at low
temperatures. We suggest that the changes induced by the oxygen isotope
exchange are caused by an increase of the charge-ordered phase in $^{18}$O
samples.
