Actually, Osamu Dazai becoming the boss did not change much. The only difference from Ogai Mori's administration was that:
Chuya Nakahara had a lot more paperwork to review.
In the first year of Osamu Dazai's leadership, under his guidance, the Port Mafia successfully swallowed countless small organizations, increasing its overall strength to 1.5 times its original level.
Chapter 154