Both Brandt brothers died in 1903, placing the fate of the company in the control of four people, the oldest of which, Paul-Emile Brandt, was only 23 years of age. Following a merger with Tissot in 1930 a new parent company, SSIH, Société suisse pour l'industrie horlogère SA, Geneva, was created. This group eventually grew to over 50 companies including, Lanco, Lémania and Hamilton. Eventually SSIH became the third largest producers of finished watches and movements in the world. During this period Omega produced some of its most collectible vintage watches Omega is famous for today.

Omega Constellation watches, the Omega Speedmaster chronograph, the Omega Seamaster waterproof sports watch and Seamaster Diver watches were all developed during this period. Omega also made a name for itself proving military and pilots watches during World War II.