In order to clarify the distribution and formation of Okhotsk Sea Intermediate Water (OSIW) an isopycnal climatological data set based on all of the available historical observations is developed and examined. The isopycnal maps clearly show that there are two direct ventilation sources for OSIW: Dense Shelf Water (DSW) with cold and fresh properties influenced by sea ice formation in the northwest shelf region and Forerunner of Soya Warm Current Water (FSCW), which has warm and saline properties originating in the Japan Sea. The cold and fresh water extends southward from the northwest shelf to the shelf slope off Sakhalin Island. It suggests that DSW is transported southward by the East Sakhalin Current. The DSW enters the Kuril Basin and mixes with FSCW and Western Subarctic Water (WSAW) originating from the North Pacific to form OSIW. The annual mean production rate of DSW estimated from the total volume of DSW over the shelf in spring and summer is 0.67 Sv (1 Sv = 106 m3/s). The annual mean volume transport of FSCW is estimated to be 0.08 Sv on the basis of its cross-section area and the current speed. Assumption of isopycnal mixing yields a mixing ratio of 1:1:0.1 among DSW, WSAW, and FSCW to form OSIW. We estimate the annual production rate of OSIW to be 1.4 Sv; the corresponding OSIW's renewal time is 7 years. Continued assumption of isopycnal mixing yields a mixing ratio of 0.6:0.4 between WSAW and OSIW to form Oyashio Intermediate Water (OYIW).