Microporous activated carbon fibers consisting of a disordered network of micrographites provide a micropore random network available for the adsorption of a large amount of gaseous materials. We investigate magnetic properties of oxygen molecules condensed in the micropore as a molecule-based random magnet. Magnetic susceptibility shows Curie-Weiss temperature dependence with the Weiss temperature of about -20K at low oxygen concentrations below 1% of the saturation concentration. The increase in the oxygen concentration makes the development of the second contribution to the susceptibility that has a broad hump around 60K. The magnetic behavior is explained in term of the formation of antiferromagnetic oxygen clusters on micrographitic domains, where the internal and marginal oxygen molecules in the cluster contribute to the susceptibility with a short range order hump and the Curie-Weiss susceptibility, respectively.