In the era of Industry 4.0, the key factor for the success of a company is the cooperation among its different departments in order to share knowledge and information, thus optimizing time and cost. Three core IT systems are usually present in companies: Product Lifecycle Management (PLM), Manufacturing Execution system (MES) and Enterprise Resource Planning (ERP). PLM manages product and process design information, MES monitors and controls the execution of manufacturing process on the shop-floor, and ERP tracks business resources and the status of business commitments. This paper describes the integration process to optimize the engineering, production, business and management activities in a manufacturing company that operates according to the One-of-a-Kind Production (OKP). It explains the architecture of such integration as well as the methodology applied for the customization of the PLM, MES and ERP systems available in the company. Moreover, the paper highlights the specific solutions developed to integrate the data management of the different systems as well as the IoT technology supporting the data communication.