Compacted graphite cast iron (CGI) is a potential alternative to lightweight alloys. It is accepted that CGI can effectively reduce the automobile total weight by virtue of its high strength and its high thermal conductivity giving an opportunity to designers to design components with thinner sections. However, the successful casting process to produce this material requires detailed information for the prediction of microstructure and accurate casting process knowledge. In this paper, compacted graphite with four different cooling rates and with different Mg/S ratios ranging from 2/3 to 7/1 has been produced by using a plunger technique. The oxygen potential of all ranges has been measured by using a special oxygen potential measuring device equipped with a vibrating oxygen measurement sensor. The relationship between the oxygen potential and the graphite shape was investigated and the oxygen potential range to produce compacted graphite iron was established. It has been demonstrated that the oxygen potential measurement is a valuable method to evaluate the potential of cast iron melts to produce compacted graphite iron.