Argon and nitrogen are widely used as shielding gas in additive manufacturing. However, the high temperature in localized melting area and the release of oxygen in the row materials can cause the difference of oxygen content between melting zone and shielding environment. Laser-induced breakdown spectroscopy is proposed to measure the trace oxygen content in argon and nitrogen near the melting zone. The experimental calibration curves are obtained from laser-induced spectra of oxygen/argon or oxygen/nitrogen mixture gas with different oxygen contents combined with the ratio of oxygen spectral line intensity to continuous background strength. Detection limits of oxygen concentration are determined to be 31×10-6 in argon and 41×10-6 in nitrogen. By analyzing the formation and decay characteristics of the plasma, we find that the electron temperature of argon plasma is higher than that of nitrogen plasma and the decay of argon plasma is slower than that of nitrogen plasma. Therefore, under the same experimental conditions, spectral line intensity and signal-noise ratio of argon plasma are stronger than that of nitrogen plasma, which lead to lower detection limit of the content of oxygen in argon.