The impacts of climate change of Osse-Ossiomo River Basin in the South-West of Nigeria under different climatic Scenarios were investigated using information on rainfall - temperature for forty years (1961 - 2000). Discharge information of Osse and Ossiomo rivers was collected for the period from 1989 to 1994. A number of water resources development schemes including hydroelectric and water supply projects have been planned in the southwestern of the river system of Nigeria. Results revealed fluctuating rainfall pattern with great uncertainties in the mid-1980s. Temperature shows an increasing trend and the highest temperature of 37oC was obtained in 1998 during the 40-year period. There is a strong evidence of global warming using the index of temperature in the drainage basin. River discharge also indicated fluctuating trends from year to year in the decades with records. It was concluded that the river discharge pattern of Osse-Ossiomo River Basin exhibited similar behaviors as other drainage basins of the world. Climate change has impacted on the river discharge of Osse-Ossiomo. This has implication on water security, sanitation and health of the inhabitants in the river basin. Most people were forced to resort to various sources for the numerous uses of water. Statistical tests had shown that the changes in rainfall temperature and rainfall discharge were significant at the 5% level.