Ogilvy picked up the brief - understood to carry fees of more than £500,000 - after a pitch process that began last November.
Nortel suspended several PR programmes in Europe, the US and Asia following its decision to repitch the account. The agency-of-record was Fleishman-Hillard.
It is understood five agencies competed for the business, including F-H.
Ogilvy will handle a wide-ranging brief covering corporate communications, trade and technical PR and media and analyst relations.
The agency reports to Nortel director of marketing and comms Sue Rizzello.
'With what has been going on in the telecoms sector in the past 18 months, our company has refocused and reshaped itself,' said Rizzello. 'It seemed like the right time to make a change with some fresh thinking and a new team who can move forward with us.'
Ogilvy director Barbara McCall has been named as lead consultant on the business, which is being led out of the agency's UK headquarters.
The programme has been expanded to cover European regions previously not targeted, including Benelux, Switzerland, Greece and parts of Scandinavia.
Ogilvy will look after these areas alongside Nortel's core markets: Iberia; France, Middle East and Africa; southern central and northern Europe.
'We have expanded the PR activity to be more flexible and meet the needs of the business today, as we have got increasingly strong business in countries outside our core markets,' said Rizzello.
She added the PR work will support Nortel's focus on returning the firm to profitability.
Last October Nortel announced it was halving its worldwide workforce from 93,000 to 45,000 as part of a major restructure.