An increasing number of research has been investigating the development of maturity models for supply chain management with special reference to the Sales and Operations Planning process (S&OP). Maturity models represent stages applied to define, implement and improve processes. Goals and benefits can be achieved at more advanced stages of process maturity. This study assesses the maturity level of the S&OP process in the complex environment of a company in the oil and gas industry through a multi-method approach. It carries out a systematic review of the literature and a single case study. After describing the company s S&OP process through Thomé et al. s (2012b) framework, the authors select and apply the Grimson and Pike s (2007) maturity model using a triangulation between document analysis, the development and application of a questionnaire, and direct observation. Among the five maturity dimensions, four (meetings and collaboration, measurements, information technology and S&OP plan integration) are in Stage three (standard S&OP) of Grimson and Pike s model and one dimension (organization) is in Stage four (Advanced S&OP). In order to advance from Stage three to four, or eventually to five, it is recommended the dissemination of the process to all the workers of the company as a tool for maximizing profits. Additionally, it is recommended to evaluate the following points: the involvement of the main customers in the forecasting process; the development of a S&OP guide; the establishment of profitability measurements associated with the S&OP effectiveness measurement; the use of an integrated and optimized system linked to SAP; and the simultaneous implementation of an integrated, continuous and optimized planning process for demand and supply. As practical contributions, it is expected that, by analysing the critical aspects and strengths identified and implementing the improvement actions suggested by this research, the company will be prepared to move to the next stage. Furthermore, the current research aims to narrow the gap between theory and current practice in the field with studies on S&OP processes in the multi-products complex environment of an oil and gas industry. For future work it is recommended to apply this methodology to assesses the maturity of S&OP in other complex environments in the oil and gas companies to strengthen the study s external validity.