Previous studies of haze formation in the atmosphere of the early Earth have focused on N_2_/CO_2_/CH_4_ atmospheres. Here, we experimentally investigate the effect of O_2_ on the formation and composition of aerosols to improve our understanding of haze formation on the Neoproterozoic Earth. We obtained in situ size, particle density, and composition measurements of aerosol particles produced from N_2_/CO_2_/CH_4_/O_2_ gas mixtures subjected to FUV radiation (115-400nm) for a range of initial CO_2_/CH_4_/O_2_ mixing ratios (O_2_ ranging from 2ppm to 0.2%). At the lowest O_2_ concentration (2ppm), the addition increased particle production for all but one gas mixture. At higher oxygen concentrations (20ppm and greater), particles are still produced, but the addition of O_2_ decreases the production rate. Both the particle size and number density decrease with increasing O_2_, indicating that O_2_ affects particle nucleation and growth. The particle density increases with increasing O_2_. The addition of CO_2_ and O_2_ not only increases the amount of oxygen in the aerosol, but it also increases the degree of nitrogen incorporation. In particular, the addition of O_2_ results in the formation of nitrate-bearing molecules. The fact that the presence of oxygen-bearing molecules increases the efficiency of nitrogen fixation has implications for the role of haze as a source of molecules required for the origin and evolution of life. The composition changes also likely affect the absorption and scattering behavior of these particles but optical property measurements are required to fully understand the implications for the effect on the planetary radiative energy balance and climate.