Oxygen is an element that is first purposely brought into the steel melt to remove some unwanted elements or to reduce their concentration (oxidation). In the made cast steel there is on the contrary necessary to reduce the oxygen content with the use of deoxidation to such a level in order to avoid a reaction with carbon with the formation of CO bubbles. Concentration of oxygen in steel before casting is given, in particular, by the manner of metallurgical processing and the used deoxidation process. Oxygen is found in molten steels both as chemically bound in the form of oxides and in the form of oxygen dissolved in the solution – the melt. Chemical composition of the melt strongly influences the activity of oxygen dissolved in the melt and further on the composition of oxidic inclusions forming in the melt during the reaction with oxygen. In the Fe-C-Cr-Ni based alloys in the reaction with oxygen greatly participates also chrome, whose products are often in solid state and they are the cause of forming such defects as e.g. oxidic films.