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, fossil fuel-based, or bio-based feedstock. 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. The use of a mass balance approach which attributes the pounds of pyrolyzed plastic products derived from pyrolysis oil to any output stream of a given unit has been developed, which permits ISCC certification agency approval.