In the present paper a new concept towards O-CMOS technology is presented substantiating the importance of the semiconductor/dielectric interface for charge carrier transport in organic semiconductors. It will be demonstrated that by controlling the interface properties of either SiO2 or PMMA, unipolar p- and n-type OFETs can be realized using a single organic semiconductor and even a single metal for source and drain contacts. Two dielectric/semiconductor interface modifications are considered for the realization of complementary OFETs on the basis of pentacene, otherwise known for its exclusive hole transporting properties. Selective modification of the SiO2 dielectric interface with traces of vacuum deposited Ca, allows for electron transport in pentacene and the realization of complementary pentacene OFETs on a single substrate. By this technique electron traps are removed due to a reaction of atomic Ca with oxygen from available hydroxide groups, resulting in the formation of an oxidized Ca layer. In a second approach, it is demonstrated that by selective UV treatment of a PMMA dielectric surface, unipolar n-type pentacene OFETs can be converted to unipolar p-type by the introduction of electron traps in the form of -OH and -COOH groups at the PMMA interface. Both methods allow for the realization of CMOS organic inverter stages with decent electrical properties.