OneUnited Bank has announced plans to move its national headquarters from its current office in Boston's Financial District to a new larger location in Roxbury or Dorchester. Although just 7 percent of OneUnited Bank's $356 million in deposits are held in Massachusetts, the expansion signals the bank's growing emphasis on its Boston-based online and mobile banking team. OneUnited Bank President Teri Williams said an expanded headquarters in Boston fits OneUnited’s stated mission to help underserved banking customers. “It represents the right kind of challenge for an institution like us,” she said. “It is the No. 1 city for income inequality as a country. Given our mission, our view is that Boston needs us. And we need Boston to figure out how [we] can solve this problem.”