Vertical organic transistors are an attractive alternative to realize short channel transistors, which are required for powerful electronic devices and flexible electronic circuits operating at high frequencies. Unfortunately, the vertical device architecture comes along with an increased device fabrication complexity, limiting the potential of this technology for application. A new design of vertical organic field-effect transistors (VOFETs) with superior electrical performance and simplified processing is reported. By using electrochemical oxidized aluminum oxide (AlOx) as a pseudo self-aligned charge-blocking structure in vertical organic transistors, direct leakage current between the source and drain can be effectively suppressed, enabling VOFETs with very low off-current levels despite the short channel length. The anodization technique is easy to apply and can be surprisingly used on both n-type and p-type organic semiconductor thin films with significant signs of degradation. Hence, the anodization technique enables a simplified process of high-performance p-type and n-type VOFETs, paving the road toward complementary circuits made of vertical transistors.