This disclosure relates to the production of chemicals and plastics using pyrolysis oil from the pyrolysis of plastic waste as a co-feedstock along with a petroleum-based or fossil fuel co-feed, or as a feedstock in the absence of a petroleum-based or fossil fuel co-feed. A mass balance accounting approach is employed to attribute the pounds of pyrolyzed plastic products derived from pyrolysis oil to any output stream of a given unit, which permits assigning circular product credit to product streams. In an aspect, the polymers and chemicals produced according to this disclosure can be certified under International Sustainability and Carbon Certification (ISCC) provisions as circular polymers and chemicals at any point along complex chemical reaction pathways.