The present invention provides an oxygen selective adsorbent containing oxides of BaxSr(1−x)Mgy(CO3)(1+y) or BaxSr(1−x)CO3 particles, increasing transition oxygen partial pressure, and representing high thermal stability and excellent oxygen sorption cavity, by adding another metal such as Sr to Ba which is active element for oxygen adsorption, so as to be capable of desorbing oxygen under lower vacuum even at the same operating temperature than the existing oxygen selective adsorbent; and a preparation method thereof.