Feasibility of polarized phosphorescent organic light-emitting devices (OLEDs) had been previously demonstrated by combining a discotic Pt(II) complex with a glassy-nematic oligofluorene host to form a mesogenic host–guest phosphorescent emitting system. Previous photophysical studies suggested that in the host–guest film, the Pt(II) complex tended to aggregate into columnar stacks, exhibiting metal–metal-to-ligand charge transfer (MMLCT) emission. Both host molecules and guest aggregates in the host–guest films could be oriented by a conductive alignment layer, giving rise to polarized phosphorescence from the Pt(II) aggregates. Nevertheless, film morphologies and nanostructures of the mesogenic host–guest systems have remained to be elucidated. In this work, grazing incidence X-ray scattering (GIXS) was carried out to analyze nanostructures in both neat films of the discotic Pt(II) complex and mesogenic host–guest mixture films. In addition, confocal laser scanning microscopy (CLSM) was also utilized for visualization of the morphologies of mesogenic host–guest systems. The columnar axes of nanostructured Pt(II) stacks lying on the alignment-treated surfaces were found to be preferentially oriented perpendicular to the rubbing direction, which is responsible for the observed linearly polarized phosphorescence.