► A SiO 2 layer was grown on the heavily doped p-type Si using a dry oxidation process. ► The effect of an O 2 gas flow on the quality of thermally growing SiO 2 was examined. ► A low O 2 gas flow led to the formation of the intermediate oxygen deficient oxide. ► The significant gate leakage is the major source of the drain offset. This paper presents an analysis of the effect of the SiO 2 /Si interfacial property on output (transfer) characteristics of organic thin film transistors (OTFTs). A SiO 2 layer was grown on the heavily doped p-type Si wafer using a dry oxidation process as a gate oxide layer. The electrical conduction investigations suggest that the leakage behavior is governed by the poor insulation of the oxygen deficient oxide between SiO 2 and Si. From the observed result, the relationship between gate leakage and output (transfer) characteristics of OTFTs was discussed.