William L. Rosoff, the former general counsel of Marsh & McLennan Companies Inc., has joined O’Melveny & Myers as a senior partner.

Rosoff was the chief lawyer for Marsh, the world’s largest insurance brokerage firm, from October 2000 until November 2004, when he resigned in the wake of New York Attorney General Eliot Spitzer’s civil suit charging the firm with rigging bids for insurance contracts and steering clients to insurers that paid it kickbacks.