  A-site ordered manganites $R$BaMn$_{2}$O$_{6}$ ($R$ = La, Pr, Nd, Sm) were
synthesized and the oxygen isotope effect in them was studied. It was found
that the substitution of $^{16}$O by $^{18}$O has caused increases in both the
Neel temperature ($T$$_{N}$) and the charge ordering temperature ($T$$_{CO}$)
and an decrease in the Curie temperature ($T$$_{C}$). The isotope exponent is
small on $T$$_{C}$ and $T$$_{CO}$ compared with that in A-site disordered
manganites such as La$_{1-x}$Ca$_{x}$MnO$_{3}$, which may be caused by weaker
electron-phonon coupling in A-site ordered ones. But the isotope exponent of
$T$$_{N}$ is large for $R$ = La. We didn't observe the large oxygen-isotope
shift that occurs in NdBaMn$_{2}$O$_{6}$, where a multicritical point is
believed to appear near $R$ = Nd, and strong fluctuation due to the competition
between ferromagnetic metal and charge ordering phase should be present.
