Onda is a town in eastern Spain, in the province of Castelló, part of the autonomous community of Valencia. It has 24,859 inhabitants. The S…Onda is a town in eastern Spain, in the province of Castelló, part of the autonomous community of Valencia. It has 24,859 inhabitants. The Sitjar dam, located close to Onda, is an important feature for the irrigation of the nearby fields with water from the Millars River.