Two major parts of work are presented in this dissertation. First, Optically Detected Magnetic Resonance (ODMR) techniques were used to study the small molecular OLEDs and films. The first work is ODMR (PLDMR, ELDMR, and EDMR) studies of classical tris-(8-hydroxyquinoline) aluminum (Alq3)-based fluorescent OLED, in which the buffer layers are AlOx and CsF. The second ODMR work is on the x% Pt octaethyl porphyrin (PtOEP):Alq3-based phosphorescent OLED (Ph-OLED). The second part of work is a combinatorial study of exciplex formation at two promising high glass transition temperature organic materials.;In ODMR studies of fluorescent OLEDs and films, low temperature enhancing resonance were observed and attributed to polaron quenching model. Very different room temperature enhancing resonances were observed for OLEDs with different buffer layer. The quenching resonance is attributed to magnetic enhanced formation of bipolarons (dianions) at organic/cathode interface.;In ODMR studies of Ph-OLEDs, we found the quenching of triplet excitons (TEs) by polarons is much less important than that of singlet excitons (SEs). A rate equation model is developed to explain experimental results. We also observed evidence of formation of bipolarons in the bulk of the phosphorescent dye doped layer.;Exciplex emission formed between organic/organic interface was observed in OLEDs made by sliding shutter technique. The efficiency of exciples is low and a method of reducing exciplex and thus enhancing device efficiency is shown. By eliminate exciplex formation, high efficient blue OLED is fabricated.