The station has four island platforms serving eight tracks. Platforms 1 to 4 are for the Yamanote Line, and 5 to 8 are shared by the Saikyō Line, the Shōnan-Shinjuku Line, and the Rinkai Line. Ōsaki is one of the stations on the Yamanote Line loop where trains are put into and taken out of service. It therefore has four tracks (two in each direction) for the Yamanote Line so as not to interfere with continuing trains (trains go several rounds before being taken out). Chest-high platform edge doors were introduced on the Yamanote Line platforms from 22 December 2012.