Abstract: Objective The aim was to illustrate the formation mechanism of carbon steel lap fillet joints fabricated by oscillation laser welding with filler wire, elucidate the relationship between the microstructure and mechanical properties of welded joints, and enhance the reliability and stability of welded joints in operating environments. Methods Based on the oscillation laser welding with filler wire, the effects of oscillation amplitude, laser power, and welding speed on the weld formation, macro-and micro-structure, and mechanical properties of low carbon steel joints in lap fillet configuration were systematically investigated. Results The comparatively small oscillation amplitude and appropriate welding speed and laser power were beneficial to the enlargement of weld width and weld height. Increasing oscillation amplitude, or increasing welding speed and laser power, could minimize the wedge-shaped notch near the weld root. However, increasing oscillation amplitude intensified the inhomogeneity in the laser power distribution and hence induced the formation of layered inhomogeneous microstructure at the weld root. During the lap-shear testing, larger wedge-shaped notch, lower weld height, and the layered inhomogeneous microstructure caused fracture within the weld, decreasing the weld toughness. Conclusion Appropriately increasing the laser oscillation amplitude while simultaneously enhancing both laser power and welding speed could effectively mitigate severe wedge-shaped notches, insufficient weld height, and heterogeneous weld microstructure. This optimization prevented fracture initiation within the fusion zone and improved the energy absorption capacity of joints during lap-shear testing.