The deal with Telefonica will potentially take oneSearch to more than 100m consumers in Ireland, Latin America and the UK, through Telefonica's local subsidiaries, which include the O2 network.
Julio Linares, Telefonica's general manager for coordination, business development and synergies, said: "Partners are essential to our mobile strategy and these key relationships, like the one we are announcing today with Yahoo! will continue to evolve and deliver state of the art internet experiences to our customers at home or on the go."
The two companies hope to increase the usage of mobile internet services through their partnership and also hope to benefit from new revenue streams from mobile advertising.
Marco Boerries, executive vice-president of Yahoo!'s connected life division, said: "Yahoo!'s new partnership with Telefonica furthers our leadership in mobile and demonstrates our commitment to working with partners to enhance the internet experience for customers on their mobile phone."