The magnetic susceptibility chi of O-2 adsorbed in micropores of activated carbon fibers (ACFs) having different micropore widths was measured at a wide temperature range of 1.7-100 K. The micropore structure of ACF was determined by the analysis of the N-2 adsorption isotherm at 77 K. The adsorption isotherm of O-2 on ACF at 77 K was determined and it was shown that O-2 is adsorbed in the micropore in a similar way to N-2 from the Dubinin-Radushkevich analysis of O-2 and N-2 adsorption isotherms. The chi-T relation suggested the presence of three kinds of O-2 molecular states in the micropore, that is, the isolated, the cluster, and condensed states; their stable region changes with the fractional filling and the temperature. Although the potential energy difference of the O-2-surface interaction of different pore systems is only 150 K by the potential calculation, the magnetic susceptibility measurement can sufficiently evidence distinctly the presence of different states of O-2 molecules in the micropore.