<p>The concern about melt pool stability has stimulated the development of oscillating laser welding process to overcome the spatter. This study focused on the melt pool stability of oscillating laser welding, and three oscillation modes (non-oscillation, sinusoidal and circular oscillation) were investigated via experiment and numerical simulation. The experimental results showed that circular oscillation mode had the best weld formation and the most stable welding process. Two mechanisms of spatter formation are revealed based on melt pool dynamics: firstly, the spatter is caused by heat accumulation effect, and recoil pressure and Marangoni force are the main driving forces; secondly, it is formed by the keyhole motion inertia-induced strong convection effect. In circular oscillating welding, the melt pool stability was dependent on the well-distributed laser energy, steady keyhole motion and regular vortex, and quantified by velocity variance, which is improved by 4.5 and 2.5 times than the non-oscillating and sinusoidal oscillating welding, respectively. This study clarified the melt pool physics of oscillating laser welding and provides guidance for its process control.</p>