The "seeding" phenomenon associated with the supersonic expansion of gaseous mixtures into a vacuum leads to cooling of the molecular degrees of freedom and also to alignment of the rotational angular momentum. Exploiting magnetic analysis of supersonic seeded beams of O-2 from various gaseous mixtures with rare gases and from air near atmospheric pressure, we demonstrate that the phenomenon naturally leads to exclusive population of the ground rovibrational triplet state and also to strong rotational alignment. The observation is also made that the alignment depends on the final speed, so that within the same beam, molecules traveling faster show much greater alignment than slow ones. The dependence of the effect on the gas carrier and on pressure and composition of the mixtures has also been investigated.