We report the importance of metamagnetism for the giant magnetocaloric effect in lanthanum manganites and the key role of oxygen deficiency for this phenomenon. It is found that La0.7Ca0.3−xSrxMnO3−δ samples with x = 0.0, 0.025, 0.05, 0.075, 0.1, and 0.125 are orthorhombic (O) phase, while those with x = 0.15, 0.175, 0.2, and 0.3 are rhombohedral (R) phase at room temperature. The samples possess relatively high oxygen deficiency δ. With increasing x, the oxygen content increases from about 2.92 to 2.94 in the O phase and from 2.93 to 2.97 in the R phase, while it decreases in the vicinity of the O-R phase transition. The Curie temperature increases with increasing x in both the structural phases and shows a positive jump around the O-R phase transition. The x = 0 sample shows S-shape magnetization, which is typical for metamagnetic materials. The S shape is continuously suppressed with increase in Sr and disappears for x ≥ 0.075. Interestingly, the magnetic entropy change, adiabatic temperature change, and refrigerant capacity achieved for the present samples are about 20%–50% larger than those previously reported for manganites prepared using different processing routes. The large magnetocaloric effect in the present samples originates from the sharp change in magnetization with respect to temperature at the Curie temperature and the presence of S-shape magnetization near this transition temperature caused by the oxygen deficiency.