Vodafone, flush with cash from the sale of its US joint-venture stake to partner Verizon for $130 billion, said the Ono deal was a "significant opportunity" to create Spain's top integrated services provider.
"The combination of Vodafone and Ono creates a leading integrated communications provider in Spain and represents an attractive value creation opportunity for Vodafone," said chief executive Vittorio Colao in a statement.
"Demand for unified communications products and services has increased significantly over the last few years in Spain, and this transaction ... will accelerate our ability to offer best-in-class propositions in the Spanish market.
"We look forward to welcoming the management and employees of Ono to Vodafone and working together to serve our customers across Spain."
The transaction, worth the equivalent of £6.0 billion, is expected to complete in the third quarter of this year but remains subject to regulatory approval.
Ono, which has about 1.9 million customers, has struggled to compete in Spain with Telefonica, Jazztel and other firms that bundle in-home services.
Vodafone meanwhile has about 14 million customers in Spain, but faces fierce competition.
"This transaction reflects Ono's attractive position as Spain's leading provider of high speed broadband, premium pay-TV and fixed communications said Jose Maria Castellano Rios, chairman of the Ono board of directors, in Monday's statement.
"As part of Vodafone, Ono will continue to seize new growth opportunities and deliver the quality that our customers expect.
"The enlarged business is also expected to drive innovation in the Spanish telecommunications industry."
The statement meanwhile followed weekend media reports that Vodafone had reached a deal.