Atmospheric pressure glow discharge (APGD) is abundant with active species, and a vacuum device is dispensable for APGD. Due to these good merits, APGD has extensive application prospects in material surface modification, biomedical application, pollutant treatment, and so on. In application research for APGD, oxygen-containing active species, such as OH radicals and O atoms, play an important role. Unfortunately, the influence of oxygen content in working gas on the concentration of the produced oxygen atom is not very clear up to now. Aim to this status, and a diffused atmospheric pressure plasma is excited by a direct-current voltage with a needle-mesh geometry, which is operated underwater of deionized water and with argon/oxygen mixture used as working gas. Discharge image presents three distinct regions, including anode glow, negative glow, and a positive column between them. The presence of these characteristic regions suggests that the discharge is operated in a glow discharge regime. By optical and electrical methods, results show that both gap voltage applied between the two electrodes and light emission signal is time-invariant, that is to say, the discharge operates in a continuous mode, other than the pulsed mode. Moreover, the voltage-current curve of discharge has a negative slope at low current and voltage stabilizing at high current, revealing that the discharge operated underwater is in a subnormal glow discharge regime at low current, and in a normal glow discharge regime at high current. By optical emission spectroscopy, spectrum collected in the range from 250 to 900 nm contains spectral lines mainly in the range of 680 to 900 nm, which are composed of Ar Ⅰ and O Ⅰ (777.4 and 844.0 nm). In addition, an OH line with weak intensity is also observed at 308 nm. With increasing O2 content of working gas, the intensity of Ar Ⅰ(750.4 and 763.5 nm) monotonously decreases. However, the intensity of O Ⅰ increases firstly, reaches a maximum with an oxygen content of 1.5%, then decreases with increasing O2 content. In order to analyze this phenomenon, the concentration of oxygen atom is then investigated as a function of oxygen content in working gas by using an intensity of O Ⅰ at 777.4 nm ratio to intensity of Ar Ⅰ at 750.4 nm. Results indicate that oxygen atom concentration with increasing oxygen content of working gas has a similar trend with O Ⅰ intensity, that is to say, oxygen atom concentration increases firstly and then decreases with increasing O2 content, and its maximum is reached with an oxygen content of 1.5%. Finally, a qualitative explanation is given through analyzing the generation process of oxygen atoms and the loss procedure of electrons attached by oxygen molecules. These results are of great significance for the application of APGD.