We explore the reaction pathways during the hydrocracking of oxygen-containing waste plastics (polymethylmethacrylate or polyethylene terephthalate) blended with vacuum gasoil (VGO). Reactions are performed in a semi-batch reactor at 400–420 °C, 80 bar, for 300 min, 10 wt% polymer/VGO, 0.1 catalyst/feed weight ratio and using a Pt-Pd/HY catalyst. The gas, liquid and solid product compositions are resolved using bidimensional chromatography (GC × GC), nuclear magnetic resonance (NMR), and Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS). The results reveal a synergistic transformation of VGO and polymer blends, with >60% fuel selectivity, > 90% plastic conversion and > 70% heavier fraction removal. The obtained naphtha contains 35–40 wt% isoparaffins and 20–25 wt% monoaromatics, ideal for gasoline blending. We provide a detailed molecular-level description of the product fractions leading to the global reaction mechanisms of these complex reactions.