La0.8Sr0.2Ga1 − xCoxO3 − δ (0.1 ≤ x ≤ 0.9, given the nomenclature LSGC1 to LSGC9, respectively), LSGC, was synthesized by solid state reaction method and its crystalline structure, electrical conductivity, and thermal expansion were characterized, respectively. A limiting current oxygen sensor was prepared with La0.8Sr0.2Ga0.8Mg0.2O3 − δ (LSGM) solid electrolyte and La0.8Sr0.2Ga0.1Co0.9O3 − δ (LSGC9) dense diffusion barrier. Influences of temperature (T), oxygen concentration (x(O2)), and thickness of dense diffusion barrier (L) on sensing characteristics of the limiting current oxygen sensor were investigated, respectively. The results show that LSGC belongs to perovskite structure and its electrical conductivity increases with increasing Co content x. Oxygen loss of LSGC3, LSGC5, and LSGC7 on the lattice occurs at high temperature and a transition from semiconductive to pseudo-metallic behavior was observed for LSGC3, LSGC5, LSGC7, and LSGC9. Thermal expansion coefficient (TEC) of LSGC increases to a maximum at LSGC7 and then decreases with increasing x in a temperature range of 300–1000 °C. The limiting current oxygen sensor exhibits excellent sensing characteristics. LogIL depends linearly on 1000/T, IL depends linearly on x(O2), and IL decreases with increasing L.