This year A&O lost banking partners Davide Mencacci to Linklaters and Giancarlo Castorino to Italian boutique MBL & Partners.
In response the firm transferred partner Paul Flanagan from London to Milan, but Sennhauser is the first high-profile hire for the new-look team.
Sennhauser has been a partner at Freshfields for seven years and specialises in structured finance transactions, debt issues and real estate finance.
His defection will come as a blow to Freshfields, which has seen four Italian partners walk out of the door this year.
The departures have included Milan IP partner Matteo Orsingher, who quit the firm to set up his own practice.
The Milan finance practice at Freshfields is left with three partners.