In the heat treating process, it is important for parts to be cooled uniformly in order to reduce distortion while obtaining target hardness. However, it is difficult to cool the parts uniformly due to the presence of the vapor blanket stage in oil quenching. In this study, we investigated the influences of cooling unevenness in various heat treatment conditions, such as the types of oils having different vapor stage lengths, and using varying agitation and oscillation rates with these oils. We found that cooling unevenness was decreased by using the quenching oil with a shorter vapor blanket stage length. Additionally, the cooling unevenness was decreased by agitation and oscillation compared to quenching without agitation. In conclusion, we found selecting a quenching oil with the shortest vapor blanket stage length and combining agitation and oscillation was the most effective method in reducing the distortion of parts after oil quenching.