The paper presents asymptotic methods for solving the problem of small harmonic oscillations of a flat contour immersed in an incompressible viscous liquid. In the case of large values of the dimensionless viscosity parameter, asymptotic approximations up to the third order have been obtained. In the case of small values of this viscosity parameter, the main term of the asymptotic of the hydrodynamic force on an arbitrary smooth contour was constructed and its form was proved not to depend on the shape of the contour. The results obtained were confirmed by an example of the problem of oscillations of an elliptical cylinder.