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Breakdown in oxygen, in external radio-frequency (RF) electric field is analyzed by employing a Monte Carlo simulation (MCS). Results were obtained for 13.56 MHz and distance between electrodes of 15 mm. Physical background of an oxygen RF breakdown is explained by observing time-resolved spatial distributions of electron concentration, mean energy, elastic scattering rate, ionization rate and attachment. The role of attachment is investigated in cases when these processes are included and when they are not. Especially influence of the attachment is highlighted by comparing oxygen and argon breakdown-voltage curves and spatial profiles. The electron losses induced by attachment extend the motion of the electron prebreakdown swarm much closer to electrodes to achieve a greater production; hence, spatial profiles at high values of the product pd where p is the pressure and d is the gap between electrodes, become more similar to those at the minimum of the breakdown curve. The most striking difference between the breakdown curves in argon and in oxygen is in the high increase of the breakdown voltage for high pd in oxygen. [ABSTRACT FROM AUTHOR]