The compatibilization of immiscible polystyrene/polypropylene (PS/PP) blends with virgin graphene oxide (GO-V) and GO modified with a bottlebrush reactive copolymer layer (GO-P) is reported. This practically important blend was chosen since, currently, PS and PP are recycled to a very low degree. The amphiphilic bottlebrush copolymer synthesized by us contained hydrophobic and hydrophilic side chains and was attached to the GO nanosheets via epoxy functionality. The GO modification and the introduction of GO into the blend were conducted from water. Thus, the introduction of the compatibilizing nanomaterial can be conducted during the mechanical recycling washing stage in a real-world situation. The final blend was prepared via melt mixing using an extruder. We examined the influence of GO modification and the mixing order on the blends' morphology, rheology, and mechanical properties. Thermodynamic calculations predicted a higher interfacial activity of GO nanosheets in PS/PP/ GO-P blends than that in PS/PP/GO-V blends. The morphological and rheological study assessed this prediction. It was demonstrated that the bottlebrush-modified GO-P sheets were readily driven to the PS/PP interphase. The mechanical measurements showed enhanced mechanical properties for PS/PP/GO-P blends, especially for those in which GO was first premixed with PS.