The oxygen-related defects in undoped Al0.3Ga0.7As (p-type) were characterized by deep-level transient spectroscopy (DLTS). We prepared pn(+)-junction diodes by the metal organic chemical vapor deposition (MOCVD) technique in order to study both majority and minority-carrier emission properties of the defects. Under majority-carrier filling condition, just one hole-emission peak (E) was observed around 450 K in addition to DX centers in the n(+)-layer. When the minority-carrier injection pulse was applied, an electron-emission peak (C) appeared around 300 K in accordance with the disappearance of peak E. The defect related to the peak E acts as an efficient recombination center which dominates the current-voltage characteristics of the diodes.