We have used ballistic-electron emission microscopy (BEEM) to locally inject and microscopically characterize trapped charge in metal-oxide-semiconductor structures made with Pt metal electrodes, moderately thick (similar to 25 nm) SiO2 films and p-type Si substrates. Electrons injected into the oxide conduction band produce a local suppression in the transmitted BEEM current across the oxide. This suppression depends on the voltage V-ox applied across the oxide during injection, indicating that its source is within the oxide film, and does not simply result from hot-electron damage at the metal-oxide interface. This suppression is accompanied by a local increase in the BEEM I-V curve threshold voltage, consistent with a build-up of trapped BEEM electrons in the oxide. For a given trapped charge density we find that the threshold varies systematically with V-ox, applied during I-V curve measurement. We use these variations to make estimates of the trapped charge density and approximate depth in the oxide film, and find them consistent with previous macroscopic observations. We further study the lateral extent of the suppression and the lifetime of this effect. We also measure the height of the Pt-SiO2 barrier and observe its lowering due to the image force effect.