In industrial practice, measuring and monitoring production performance is an essential task. The production plan performance is monitored by middle and top management of companies daily, weekly and monthly and make short and long-term operational and strategic decisions when necessary. One of the most common ways of measuring the performance of production and, within this, of assembly lines, is to use the Overall Equipment Effectiveness (OEE) indicator. Although companies sometimes interpret and use this Key Performance Indicator (KPI) in their own way, it is the indicator that best reflects the development of the production efficiency for a given company. A high OEE percentage means high performance, which directly increases the company's profitability. This article explores the complexity of the OEE indicator, supported by the use of a cause and effect diagram. Firstly, a literature review demonstrates scientific relevance. Secondly, the factors affecting OEE are grouped and analyzed according to the following six aspects: man, environment, method, material, machine, and measurement. Each factor is further subdivided into five groups, and then these subgroups also cover five key factors of importance for the approachability of 100% OEE. The 150 aspects listed herein, provide a complete guideline for a semi-automatic assembly line, to consistently increase efficiency in industrial practice.