Abstract: The ocean plays a vital role in determining weather and climate. The physical and chemical state of the ocean is key to determine marine habitats and the health of marine lives. Hence, it is imperative to understand the causes of variability in the ocean state across different time scales through data collected by in-situ ocean observation platforms. Further, the knowledge acquired through in-situ observation networks act as building blocks in the development of ocean state forecasting systems. Considering these aspects, ESSO-INCOIS has been maintaining various in-situ observation platforms in the Indian Ocean under the Ocean Observation Network (OON) programme. A synopsis of these networks is summarised in this article.