The magnetic susceptibility of O 2 molecules confined in slit-shaped graphitic micropore systems with the slit pore width w of 0.75, 0.96, and 1.45 nm was measured at diffrent coverages F and over the temperature range of 1.7-100 K. For all micropore systems, the adsorbed O 2 at a very low coverage showed the paramagnetism without any obvious magnetic transition, which is quite close to the paramagnetism of bulk gaseous oxygen. An antiferromagnetic behavior appeared with an increase in F. This behavior was caused not by the formation of 2D lattices or bulk solid phases but by the random magnetism for the systems of w ) 0.75 and 0.96 nm. The random magnetism indicates the cluster formation of the O 2 molecules in the nanospace. The paramagnetism and the random magnetism arise from an enhanced intermolecular interaction of O 2 in the unique potential field of the narrow slit space. The O 2 assembly in the micropores showed a different magnetic behavior dpending on the pore width. Besides the cluster formation, a phase like the θ phase, which appears in the O 2 bilayer on the 2D graphite surface, was observed for 0.4 < F < 0.8 in the micropores of w ) 1.45 nm. This suggest that the micropores with w ) 1.45 nm provide an enhanced formation of the specific 2D phase. These unusual magnetic behaviors were presumed to be caused by the special critical phenomenon in the nanospace.