Time-resolved small-angle X-ray scattering (SAXS) experiments were carried out on concentrated solutions of bottlebrush polymers exposed to an external shear flow. We followed the theological response of the sample online. If the experiment is performed in the strain-controlled mode, the rheological response of the bottlebrush polymer solution to the rotational shear shows oscillations with changes in viscosity of almost 2 orders of magnitude. The SAXS data reveal that this oscillatory response is due to a reentrant phase transition between a shear molten phase and a line hexatic phase. It is not due to shear-induced phase separation leading to shear band formation. The in-situ theological SAXS measurements allow a detailed description of the structural changes occurring in the sample during structural buildup and breakdown.