This study examines the history and development of the Ogun-Oshun River Basin Development Authority (O-ORBDA) from 1977 to 2024, focusing on its evolution, policies, projects, and contributions to water resources management, irrigation, agricultural productivity, rural development, and regional socioeconomic transformation in southwestern Nigeria. Using a qualitative historical methodology, the research draws on oral history interviews with officials, farmers, community members, local leaders, and policymakers, alongside archival records, government documents, reports, academic books, journal articles, and other secondary sources. The study is significant because it provides a detailed historical account of O-ORBDA and examines both its achievements and persistent challenges, including funding limitations, administrative difficulties, policy inconsistencies, and institutional constraints. It contributes to understanding the role of river basin development authorities in Nigeria’s development history and offers lessons for sustainable water resource management, agricultural development, and regional development.