We report on a remarkable viscosity transition of poly(p-hydroxystyrene)-graft-poly(propylene oxide-ran-ethylene oxide) (PHOS-g-P(PO-r-EO)) bottle-brush copolymers and their underlying spatial solution arrangement in aqueous solutions. The changes of morphology are measured by small-angle neutron scattering, and the rheological alterations are characterized by viscometry and video particle tracking microrheology. At room temperature core shell micelles are formed, and the dynamics of the solutions have soft colloidal nature. The viscosity passes through a pronounced maximum as temperature increases, and at temperatures higher than the transition regime the solutions end up as thin and milky-white dispersions of particulate clusters. The double temperature-thickening/temperature-thinning transition is related to the interactions and associations of the formed clusters. These findings are very useful for the design and application of thermoresponsive brush copolymers in a variety of fields.