We describe the synthesis and self-assembly of a terthiophene-oligostyrene based block co-oligomer Ter-OSa. While the self-assembly of block copolymers is normally achieved using block-selective solvents, tetrahydrofuran (THE) is a good solvent for both terthiophene and oligostyrene blocks. The self-assembly in this case arises with the nonsolvent (H(2)O) in THF/H(2)O mixtures. The absorption spectra showed a blue shift and fluorescence spectra showed quenching of fluorescence with increasing water content indicating that the Ter-OSa self-assembled to form H-aggregates. The morphology was studied using scanning electron microscopy. Depending upon the amount of water the Ter-OSa undergoes morphological transition from hollow to rigid spheres. However at higher water content the Ter-OSa formed large spheres from the fusion of smaller spheres. A mechanism of morphological transition from hollow spheres to rigid spheres is proposed.