Abstract This paper describes how an opportunity to determine residual oil saturation in-situ was captured in the Angsi field, a relatively new development in the South China Sea off the East coast of peninsular Malaysia. It describes the data required to determine fluid saturations in-situ in a well drilled through a reservoir which has been flooded by injection water, focusing on the importance of getting representative water salinity and reservoir temperature data. It also discusses how the residual oil saturation (Sor) determined in-situ compares with other Sor estimates. Residual oil saturation is a critical parameter within reservoir engineering. Sor values are often based on laboratory tests performed on core plugs. Because laboratory Sor data is sensitive to plug heterogeneities and testing method, there is often a high degree of uncertainty associated with such data. More reliable, and more accurate, Sor information can be obtained in wells that have been drilled through flooded parts of the reservoir. Capturing such opportunities requires proper planning and preparation. At Angsi, such an opportunity was captured during field development drilling. High quality in-situ Sor data was obtained in one of the major oil reservoirs. As a result, the residual oil saturation is now better defined, or less uncertain. This means there is a higher degree of confidence in the reservoir model and production forecasts generated from the model. The importance of reduced uncertainty and a higher degree of confidence in production predictions cannot be overemphasized. Such improvements are key to continued development optimization, and should ultimately lead to accelerated production and increased recovery.