Abstract: To address the problems of lack of fusion caused by uneven arc heat distribution and poor weld formation induced by molten pool sagging in narrow-gap GMAW vertical-up welding, as well as the unclear mechanism relating welding parameters to arc sidewall climbing and lack of fusion under oscillating arc conditions, oscillating arc narrow-gap GMAW vertical-up welding experiments were conducted using the matching welding wire MG-80 for 690 MPa-grade low-alloy high-strength steel. The effects of peak voltage, oscillation amplitude, and sidewall residence time on arc stability and melting dimensions were investigated. The spatial distribution of wire tip coordinates and the numerical distribution of the distance from the wire tip to the groove bottom were adopted to characterize arc stability. The minimum sidewall penetration near the groove fillet, the minimum penetration at the inner groove bottom, and the macro morphology of weld seams were used to evaluate the tendency to generate sidewall and interlayer lack of fusion. The results indicate that if parameter variations intensify the arc sidewall climbing behavior, the minimum welding penetration decreases. The increases in peak voltage, oscillation amplitude, and sidewall residence time all increase the distance from the wire tip to the groove bottom during the oscillation stage, lowering the probability of arc conduction with the groove bottom. The arc skips the transition process from the groove bottom to the groove fillet and then to the sidewall and ignites on the sidewall in advance. Consequently, the minimum penetration at both the groove bottom and the sidewall decreases, and the tendency to generate sidewall and interlayer lack of fusion increases.