If the deal goes through, it will be one of largest IPOs in the history of equity capital markets.
Michael Bon, chairman and chief executive of Orange owner France Telecom, believes it could value the company at more than euro100bn (£60bn).
Freshfields and Davis Polk & Wardwell are advising the investment banks: Dresdner Kleinwort Benson, Morgan Stanley Dean Witter and Société Générale.
The IPO, which is expected to take place by the end of the year, will be based in London, with listings in Paris and New York.
Freshfields recently scored highly in The Lawyer IPO Survey 2000, coming in just behind Linklaters & Alliance as one of the leading equity capital markets firms (The Lawyer, 18 September).
Freshfields has worked on a range of IPOs over the last year, including advising Thus, the telecoms arm of Scottish Power, on its £2.19bn flotation in November.
The firm also has relationships with a number of investment banks, which are vital to equity capital markets work, as the bulk of it comes through referrals.
While Slaughter and May has been involved in a number of key deals, it has undertaken less IPO work than Freshfields, although it too has strong relationships with a range of banks.
It is not the first time both firms have had dealings with Orange, which was taken over by Mannesmann earlier this year.
Vodafone then launched a successful hostile bid for the German telecom, but had to divest Orange to comply with competition rules.
Slaughters acted for Orange when it was taken over by Mannesmann. Freshfields was conflicted out from representing long-term client Mannesmann, but future merger partner Bruckhaus Westrick Heller Löber was instructed.
Later in the negotiations Freshfields represented the German telecom alongside Norton Rose.
Up until spring this year, the IPO market was incredibly strong, with investors showing particular interest in new economy stocks. But even now the dotcom bubble has burst, some technology companies, specifically telecoms, are still attracting interest.
Freshfields and Slaughters were unavailable for comment.