The aim of this study was the evaluation of biochar as a means of improving the performance of the composting process and the quality of the end product. For this purpose, oak biochar was added at a rate of 10% dry weight to a 50:50 mixture of green waste and the organic fraction of municipal solid waste in a full-scale composting process. The main improvements caused by biochar addition were an enhanced rate of organic matter (OM) decomposition during the bio-oxidative phase of the process (OM losses of 21 and 11% for feedstocks with or without biochar, respectively), without affecting the total OM loss at the end of the process, a significant reduction of N losses (3 and 22% of initial N for compost with or without biochar, respectively) and a larger amount of easily available P (17.4 vs. 14.9 mg L-1 for compost with or without biochar, respectively). Biochar caused a reduction or no effect on greenhouse gas (GHG) emissions, depending on the specific gas and conditions of the composting process. The results indicate that adding biochar to the feedstock mixture before composting represents an effective improvement to the traditional composting system, especially in the case of N-rich organic wastes, as it may reduce the process costs and GHG emissions, without affecting the quality of the end product.