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 London-listed Vodafone, flush with cash from the sale of its Verizon Wireless stake to partner Verizon for USD 130 billion, agreed in March to purchase Ono for USD 10 billion.
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.
Vodafone meanwhile has about 14 million customers in Spain, but faces fierce competition.
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Vodafone had last year sold its 45-per cent Verizon Wireless holding to Verizon for USD 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 pound 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.
The European Commission today 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.