We study the dynamics of paramagnetic molecular superrotors in an external magnetic field. Optical centrifuge is used to create dense ensembles of oxygen molecules in ultra-high rotational states. It is shown for the first time, that the gas of rotating molecules becomes optically birefringent in the presence of magnetic field. The discovered effect of "magneto-rotational birefringence" indicates preferential alignment of molecular axes along the field direction. We provide an intuitive qualitative model, in which the influence of the applied magnetic field on the molecular orientation is mediated by the spin-rotation coupling. This model is supported by the direct imaging of the distribution of molecular axes, the demonstration of the magnetic reversal of the rotational Raman signal, and by numerical calculations.