For demanding wire applications steel cleanliness should be very high and the inclusions inevitably found in steel should be harmless. This means strict control of inclusions' size, quantity, and composition, pursuing deformable inclusions in rolling conditions. Primary inclusions are formed during steel treatment in the ladle. Most of these are removed to the ladle slag or on the lining. However, the rest of the inclusions still remain through the successive process stages, and some new inclusions are formed during casting and solidification. Conventionally, deformable inclusions are produced by Si–Mn deoxidation resulting in MnO–SiO2–Al2O3 inclusions. This leaves, however, the oxygen content too high for demanding applications. In order to get really clean steel, the Si deoxidation needs to be strengthened by lowering the activity of SiO2 forming in steel. This can be done by bringing the steel in intimate contact with a slag containing SiO2–MnO–Al2O3 and additionally CaO and some MgO. With this kind of intensified Si deoxidation it is possible to produce steels with low oxygen content having inclusions that will elongate at rolling. In this paper thermodynamic examination of potential slag systems and compositions to equilibrate with steels having medium carbon and high silicon were scrutinized. The optimal slag composition for producing low-O steels with deformable inclusions was evaluated by using FactSage thermodynamic calculation program. The lowest SiO2 activities at the region in which slag is still liquid at 1400°C, can be found when slag composition is approximately 36–40 wt% SiO2, 6–18 wt% Al2O3, 30–40 wt% CaO, 6–8 wt% MgO, and 2–4 wt% MnO. Industrial heats using intensified Si deoxidation and slag based inclusion engineering were produced in a steel mill with 60 tons heat size. Inclusions and slag compositions were in satisfactory accordance with the theoretical examinations, though some scattering was discovered.