Oxygen isotope effect in magnetic properties of CMR manganites of the (La1- yPry)0.7Ca0.3MnO3 system was systematically studied. Substitution of 16O by 18O was found to stabilize insulating state and suppress ferromagnetic (FM) component. The AC susceptibility data revealed a pronounced temperature hysteresis. The hysterisis became more pronounced with increase of /y up to 0.75. The width of the hysteretic loops was strongly affected by the 16O-->18O substitution. The /T-/y phase diagram and its evolution due to oxygen isotope substitution are analyzed.