Little is known about fossil ostracods from deep-sea drilling sites in the marginal sea bordered by the Eurasian continent, the Korean Peninsula, and the Japanese Islands. This report presents taxonomic descriptions for the Pliocene and early Pleistocene deepsea ostracods at Integrated Ocean Drilling Program Expedition 346 Site U1426 on the Oki Ridge in the marginal sea. We found 23 species and 12 genera in discrete core samples. The ostracods constitute taxa that dwell under the Tsushima Warm Current and the Japan Sea Intermediate-Proper Water. We present taxonomic summaries of ostracod species distribution and ecology in addition to scanning electron microscopic images. In 2014, Integrated Ocean Drilling Program (IODP) Expedition 346 cored 7 sites in the marginal sea and 2 sites in the South China Sea (Fig. F1). At IODP Site U1426, four holes were cored at 903 meters below sea level (mbsl). Here, we report on the ostracod taxa found in Pliocene to Pleistocene sediments and describe them taxonomically. Following the shipboard age model (see the "Site U1426" chapter [Tada et al., 2015]), the geological age of the sediment samples ranges from 4.8 to 1.2 Ma.