The oxygen content of rare earth manganates La0.5Ca0.5MnO3, Pr0.5Ca0.5MnO3, Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 with different degrees of charge ordering instabilities is varied by controlled annealing in N-2. The oxygen deficiency increases the unit cell volume and decreases the tolerance factor. The electron transport and magnetic properties are sensitive to the deviation of the %Mn4+ variation of tolerance factor of the oxygen deficient charge ordered manganates from that of the oxygen stoichiometric manganates with same A-site ions and different (r(A)). Pr0.5Sr0.5MnO3 and Nd0.5Sr0.5MnO3 with larger deviations show different behavior while La0.5Ca0.5MnO3 and Pr0.5Ca0.5MnO3 with smaller deviations follow the behavior expected from the decreasing Mn4+ concentration. Oxygen deficiency broadens the Mn4+ concentration range of charge ordering instability in the former and suppresses in the latter. Apart from the crystal structure deformations the oxygen defects modify the electronic band structure in a complex manner.