Laser welding of structural steel encounters variable joint gaps, which may need beam oscillation or beam shaping strategies to ensure stable gap bridging. This study presents combined experimental and computational fluid dynamics (CFD) investigation into the melt pool dynamics, surface geometry, and temperature profiles of four 100 Hz beam oscillation modes (linear, circle, figure-8, and infinity) across gap distances from 0.0, 0.2, 0.4 to 0.6 mm. In-process monitoring was conducted using a thermal camera, positioned in side and rear viewing configurations, to capture transient surface temperature profiles. Post-process surface geometry and gap bridging efficacy were quantified using focus variation microscopy. Experimental results showed a physical bridging threshold that gaps up to 0.2 mm were successfully filled, whereas 0.6 mm gaps resulted in consistent bridging failure across all oscillation modes. CFD modeling resolved the fluid flow driving these geometric features, demonstrating that counter-clockwise circle oscillation induces asymmetric fluid displacement toward the trailing edge. Empirical thermal analyses confirmed that increasing gap width decreases peak surface temperatures due to less laser-material interaction time and accelerates cooling rates through steep conductive gradients into the base plate. This work provides insights for applying beam oscillation for gap bridging in laser welding process for structural steel applications.