In fact, without the ossicles, the membrane would hardly move in response to sound waves. This is because of the pressure that the cochlear fluid exerts at the oval window. Pressure is defined by force applied per unit area. The ossicles act like levers to increase the force at the oval window. They also transfer the sound force from the large surface area of tympanic membrane onto the much smaller membrane at the oval window. By increasing the force and decreasing the surface area, the ossicles hence provide the amplication in pressure sufficient to move the cochlea fluid for sound detection.
Figure 1: The connection between the tympanic membrane and the ossicles within the middle ear