The company, which counts Freshfields Bruckhaus Deringer among its main advisers, will kick off a process in the coming months, led by general counsel Justine Campbell.
The Slough-based company has also previously used firms including Olswang, SJ Berwin, Shearman & Sterling, Baker & McKenzie and Herbert Smith.
The advisers will be aware of the possible influence of O2’s parent company Telefonica on the review. The Spanish company works with firms such as Clifford Chance and Linklaters as well as Simmons & Simmons, which advised on its £17.7bn takeover of O2 in 2005. Freshfields also has a relationship with Telefonica.
O2’s decision to hold a review will also be of interest to Shoosmiths, DLA Piper, SJ Berwin, Wragge & Co and McGrigors, which have all advised the telecoms giant in the past.
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Bramwell’s departure came just two years after head of legal Richard Hastings left to become Yahoo’s European general counsel in 2004.