The rational modification of perovskite oxides (ABO_3−δ) is essential to improve the efficiency and stability of oxygen electrolysis. Surface engineering represents a facile approach to modify perovskites for enhanced performance. Through compositional design and in situ exsolution, a Ru-doped (La_0.8Sr_0.2)_0.9Co_0.1Fe_0.8Ru_0.1O_{3−​​​​​​​δ} (LSCFR) perovskite anchored with CoFe(Ru) alloy particles on the surface was fabricated for oxygen evolution reaction (OER) in this work. Experimental results and calculations indicate that Ru-doping promotes the exsolution of CoFe(Ru) from the perovskite parent. Upon exsolution in the reduced atmosphere for 3 h, the catalyst (LSCFR-3) exhibited superior OER performance with an overpotential of 347 mV and a Tafel slope of 54.65 mV·dec⁻_{​​​​​​​}¹, and showed good stability in contrast to the pristine LSCFR. The exsolution of CoFe(Ru) particles, Ru doping, and the increase of surface oxygen vacancies are responsible for the enhancement of OER performance. The findings obtained in this study highlight the possibility of controlling exsolution and composition of nanoparticles by element doping and prove that in situ exsolution is an effective strategy for designing OER catalysts.