The Wizards brought about the extra session when Chris Singleton delivered a dunk shot with 9 seconds to go in the fourth period and Washington took a 90-88 lead in overtime with 3:49 to play.
Boston then seized control with Bass putting the Celtics in front for good by hitting a free throw with 2:21 remaining. He boosted the Boston lead with a layup 25 seconds later and added a dunk with 37 seconds left that gave the Celtics a 97-92 advantage.
Boston won its second straight after losing its first two games of the season. The Wizards began the evening as one of two winless teams in the NBA.
Washington got 16 points from Bradley Beal.