The effect of oxygen isotope substitution on the alternating-current magnetic susceptibility for (La_1-yPr_y)_0.7Ca_0.3MnO₃ ceramics (y = 0, 0.25, 0.5, 0.75, 1) was studied in the temperature range 4.2-300 K. At y < 0.75, the low-temperature behaviour of these manganites is typical for a ferromagnetic metal, whereas at y close to 1, the system behaves as an antiferromagnetic charge-ordered insulator. The ¹⁶O¹⁸O isotope substitution significantly shifts the Curie point T_C, but has a smaller effect on the Néel temperature T_N. At the crossover concentration y = 0.75, the exchange of ¹⁶O with ¹⁸O gives rise to a metal-insulator transition. The temperature dependence of the magnetic susceptibility is characterized by a pronounced hysteresis at y0 or 1, and the width of the hysteresis loops is strongly affected by the ¹⁶O¹⁸O exchange. The behaviour of the susceptibility in the paramagnetic phase gives an indication of the phase separation over quite a wide temperature range near the transition point. The phase diagram in the T-y plane is obtained and its modification with isotope substitution is discussed.