The Oyashio seasonal intensification and its effect on subsurface temperature fields off the Sanriku coast are investigated, based on indices of the Oyashio current. These indices include altimeter-based eddy drifting velocities (EDV) and tide gauge sea levels at Petropavlovsk-Kamchatsky (PK), where the Oyashio upstream region is located. The annual cycles of EDV, the PK sea levels and the Sverdrup transport for the range of 40--50¿N (SV) show similar seasonal features that have (1) maximums in early winter and (2) small secondary local peaks in early summer. These indicate that the Oyashio seasonal variation is mainly governed by the Sverdrup dynamics. The early winter intensification recognized as the general Oyashio seasonal variation involves a large and rapid change from late autumn to early winter, which is roughly explained by the southward shift of westerlies. Its onset timing, which undergoes large year-to-year variability, is indeed determined by the timing of the southward migration of the westerly jet. The early summer intensification can also be explained by a barotoropic response to the intraseasonal evolution of atmospheric circulation in the Baiu (rainy) season, which is observed characteristically in East Asia and East Siberia. The seasonal cycle of subsurface temperatures off the Sanriku coast, where the Oyashio water meets the Kuroshio water, shows (1) a minimum in April and (2) a secondary local minimum in July. The cooling cycle is qualitatively explained by lateral heat advection by the Oyashio current.