The viscoelastic properties of oligomer melts (i.e., thin films) confined by two walls are studied from molecular dynamics simulations. The storage and loss moduli are calculated by shearing the upper wall periodically. The effects of film thickness, pressure, chain length, and wall-film interactions are investigated as a function of frequencies. Even if the film is thinner than 5 molecular diameters, frequency dependence of the basic viscoelastic properties follows Rouse theory. However, hardly any molecular-weight difference is seen in thin films in the high-frequency region. The loss modulus shows that viscosity decreases with increasing frequency as shown in surface force apparatus experiments. With higher pressure, the tendency becomes more noticeable and the nonlinear shear response increases and storage and loss moduli are almost the same, independent of frequency, like under the solid state. The storage and loss moduli calculated from the shear response by shearing the slider are also compared with those calculated by the Green-Kubo formula.