In 1997, Osmeña bolted Lakas NUCD to be an independent senator. In 1998, he joined the Liberal Party. Osmeña succeeded Senator Edgardo J. Angara as Chair of the powerful Committee on Agriculture and Food and the Congressional Oversight Committee on Agricultural and Fisheries Modernization (COCAFM). As Chair, Osmeña tried to block the approval of the construction of the Casecnan and the San Roque dams having discovered gross discrepancies in the costs and benefits of the projects. It was Osmeña who pointed out that the government would be investing a ridiculous amount of more than P385,000 to irrigate an additional hectare of rice paddy. The expose led to the cancellation of the proposed Balog-Balog irrigation project.