Quantum interference oscillations of electrons in a thin SiO2 layer were observed by ballistic electron emission microscopy (BEEM). With BEEM, electrons are injected across the gate of a metal-oxide-semiconductor (MOS) structure and directly into the conduction band of the SiO2. The MOS capacitor consisted of a 5 nm thick Pd film deposited on a 2.8+/-0.2 nm oxide thermally grown on Si(100). Oscillations with up to four peaks in an energy range of 0-3 eV above the injection threshold were noted. Their magnitude is of the order of 30% of the underlying BEEM current. The oscillations were most salient and their energy location repeatable at points of the sample that were previously not exposed to the electron beam. Even modest exposures caused a buildup of positive charge. This charge resulted in energy shifts, as well as a weakening of the oscillations, both of which are a consequence of the added scattering and local field inhomogeneities associated with the random distribution of the positive charge. Solutions of the Schrodinger equation that included a built-in oxide potential of 0.20 V and image force effects at both interfaces gave excellent fits to the experimental data for an effective electron mass in the oxide m(ox) = 0.63 +/- 0.09m(o). The uncertainty in m(ox) arises from an uncertainty of +/-0.2 nm in the determination of the oxide thickness by ellipsometric methods. Nevertheless, the obtained value is well above the generally accepted value of 0.5m(o).