InfoNXX has chosen the brand name "The Number 118118 to market its domestic directory service and OMD joins a list of agencies including WCRS and Naked working on the business.
Red Zebra, the agency headed by the former Orange marketing director Chris Moss, has also taken on a consultancy role for the launch of the InfoNXX service.
Although the company is forbidden from advertising until deregulation is completed in the middle of December, it is expected that InfoNXX will concentrate its resources on guerrilla marketing until then.
InfoNXX will be using the 118118 phone number, which is supposed to be the most memorable out of all the directory enquiry numbers.
The company was forced to pay £2 million to Leaf Telecom for the number as it had originally won the 118811 permutation in an Oftel draw in May.
InfoNXX, which launched in 1995, is expected to use the 118811 number for its international directory enquiry service.
The directory enquiries market has been opened up to competition following complaints that BT has a monopoly on the business - currently consumers can only access the directory enquiry service by dialling 192.
A number of new players have entered the market such as the Irish company Conduit, which will use the 118899 number while BT has been allocated 118500.
The existing 192 number, and the 153 international directory enquiries number, will run in tandem with the new numbers until they are phased out in August of next year.