The firms replace Skadden Arps Meagher & Flom, which advised Tyco on the preparatory work for the restructuring.
A&O is international counsel and McDermott is acting as US counsel (with Gibson Dunn & Crutcher doing securities filing) as the $41bn (£23.61bn) conglomerate splits into three companies: the $18bn (£10.36bn) Tyco Fire & Security and Engineered Products & Services; the $12.5bn (£7.2bn) Tyco Electronics; and the $9.5bn (£5.47bn) Tyco Healthcare.
A&O's relationship with Tyco stems from its status as lead adviser to ADT, which was acquired by Tyco in 1997.
A&O's corporate co-managing partner Mark Wippell will lead the team, with London corporate partner Julian Holgate coordinating London and Europe, which will be led by Marc Feider in Luxembourg. US partner Dan Cunningham is working on the deal in New York and Ken Aboud is acting as Asian project leader.
The head of McDermott's Washington DC corporate team Karen Dewis is leading the US effort.
The appointment should give A&O a head start as Tyco plans to streamline its Europe, Middle East and Africa operations. In the UK Tyco also uses firms including Clifford Chance, Cobbetts, McGrigors, Pinsent Masons and White & Case.