The influence of oscillatory shear deformation on spinodal decomposition (SD) processes in a blend of polybutadiene and polyisoprene has been investigated by in-situ observations of light scattering patterns. A large strain amplitude gamma(0) = 0.8, with angular frequencies omega = 0.63 and 6.3 rad/s, was imposed on the blend undergoing SD induced by a temperature jump from a homogeneous (or single phase) region to a thermodynamically unstable region. Comparing representative rates of the shear deformation (i.e., frequency omega and a maximum shear rate) with representative growth rates for SD (i.e., growth rate of concentration fluctuations in the early stage SD and that of domains in the later stage SD), the deformation used in the present study is expected to bring the system into a homogeneous state, based on an estimation in the simple shear flow case. In spite of this strong shearing criterion, SD still occurred at omega = 0.63 rad/s : the phase-separated structure is affinely deformed in harmony with the shear strain phase and grows with time on a time scale longer than the cycle of the oscillatory deformation. In contrast, no remarkable sign of SD could be observed up to 80 min after the onset of SD under oscillatory shear deformation at omega = 6.3 rad/s, although well-defined characteristic scattering of SD ("spinodal ring") can clearly be observed both in the absence and in the presence of the low-frequency shear (omega = 0.63 rad/s) at the corresponding time scale.