Dielectric barrier discharges (DBDs) offer great potential for applications such as volatile organic compounds (VOCs) conversion or plasma catalysis. For many of these applications, an admixture of molecular oxygen is important, for example, to oxidize the gases to be treated. However, oxygen addition can lead to a drastic change in discharge dynamics, which may affect conversion efficiency. The discharge may transition to a filamentary mode, which could influence plasma chemical processes or in extreme cases potentially damage the reactor. This study investigates the discharge mode of a micro cavity plasma array operated at atmospheric pressure with a helium flow (1-2slm) containing small oxygen admixtures (0-5%). A multitude of parameters as voltage, current, power, and emission are investigated for characterization. Additionally, the electric field, mean electron energy and atomic oxygen density are examined depending on the oxygen admixture. With pure helium, a homogeneous atmospheric pressure glow discharge (APGD) is observed, appearing as a quasi-continuous glow discharge. With small oxygen admixtures (0.1-1%), individual discharge pulses become visible, though they remain separated (pseudo glow discharge). At higher oxygen admixtures (>1%), a mode transition to a filamentary discharge is observed. The measurements indicate that the discharge mode, especially the individual discharge pulses, has a significant impact on conversion efficiency. This knowledge can help in the future to fine-tune the discharge mode using external parameters such as voltage waveform or frequency to optimize conversion efficiency.