In a statement, the commission said the companies' activities were "largely complementary" with Ono's main activity related to fixed line telecommunication services "whereas Vodafone is mainly active in mobile telecoms".
The acquisition marks Vodafone's desire to grow in Europe following last year's 7.7-billion-euro takeover of Kabel Deutschland, the largest cable operator in Germany, and amid fast-moving consolidation within the industry.
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.
Vodafone had last year sold its 45-percent Verizon Wireless holding to Verizon for $130 billion, clinching one of the biggest transactions in global corporate history.
The British group, which is the world's second-largest mobile operator by subscribers after China Mobile, has vowed to invest £19 billion over the next two years on networks and services by March 2016.
The Ono transaction meanwhile marks the latest consolidation in the fast-moving global cable industry.
US giant Liberty Global took over its British rival Virgin Media last year in a deal worth $23.3 billion.
The European Commission on Wednesday also approved the creation of Germany's biggest mobile company when it cleared the acquisition of E-plus in Germany, a unit of Dutch KPN, by Telefonica of Spain.