The effect of the formation of p(2 x 2) and c(2 X 2) adsorption structures on the O-2 sticking probability was studied by the Monte Carlo simulation of the O-2 chemisorption on a (100) metal surface. The model used in the simulation took into account the direct and indirect adsorption pathways and repulsive lateral interactions in the adsorption layer. The ratio between the activation energy of adsorption via the direct and indirect pathways ( E-dir/E-indir) determines the character of the structural transformations in the adsorption layer, which in turn determines the type of the S(theta) dependence. At E-dir/E-indir < 7-8, the direct pathway predominates over the indirect one. In this case, a lot of small adsorption islands nucleate to form a disordered p(2 X 2) and c(2 x 2) adsorption layers, i.e. Langmuir adsorption is observed, and S smoothly decreases with the a growth. At E-dir/E-indir > 7-8, the indirect pathway predominates over the direct one. In this case, after slow nucleation, the adsorption islands grow quickly to form ordered p(2 X 2) and c(2 X 2) adsorption layers, i.e. the island-mediated adsorption is observed and S(theta) dependence passes over a maximum.