A new passive control technique of cavity-induced pressure oscillations has been investigated numerically for a supersonic two-dimensional flow over open rectangular cavities at Mach number 1.83 at the cavity entrance. A sub-cavity on the front wall of the cavity covered by a flat plate was evaluated for the effectiveness of controlling cavity-induced acoustic oscillations. The results showed that sub-cavity is very effective in reducing cavity-induced pressure oscillations. The results also showed that the resultant amount of attenuation of cavity-induced pressure oscillations was dependent on the length and thickness of the flat plate, and also on the depth of the sub-cavity used as an oscillation suppressor.