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The safety issue caused by the dendrite growth is not only a key research problem in lithium-ion batteries but also a critical concern in alkali metal (i.e., Li, Na, and K)-oxygen batteries where a solid metal is usually used as the anode. Herein, we demonstrate the first dendrite-free K-O battery at ambient temperature based on a liquid Na-K alloy anode. The unique liquid-liquid connection between the liquid alloy and the electrolyte in our alloy anode-based battery provides a homogeneous and robust anode-electrolyte interface. Meanwhile, we manage to show that the Na-K alloy is only compatible in K-O batteries but not in Na-O batteries, which is mainly attributed to the stronger reducibility of potassium and relatively more favorable thermodynamic formation of KO over NaO during the discharge process. It is observed that our K-O battery based on a liquid alloy anode shows a long cycle life (over 620 h) and a low discharge-charge overpotential (about 0.05 V at initial cycles). Moreover, the mechanism investigation into the K-O cell degradation shows that the O crossover effect and the ether-electrolyte instability are the critical problems for K-O batteries. In a word, this study provides a new route to solve the problems caused by the dendrite growth in alkali metal-oxygen batteries.