The Celtics grabbed control in the final period, opening an 86-71 lead on Jonas Jerebko' transition 3-pointer from the left corner.
Philadelphia trailed by 11 at the break, but closed to 72-67 on Hollis Thompson's 3-pointer from the left corner. Boston then scored the final seven points of the third. Thomas, who had 13 in the quarter, nailed a 3 from the right wing in the final seconds.
Coming off a 40-42 record and a seventh-place finish in the Eastern Conference in coach Brad Stevens' second season, the Celtics entered their 70th year in the NBA loaded with a roster full of what could be described as complimentary players who could fill different roles.
Philadelphia, meanwhile, is young again. Twelve of the 15 players on the 76ers' opening roster had two years or less of NBA experience. They went 18-64 last season, and then drafted Okafor out of Duke.
Okafor had 10 points to help the 76ers open a 26-17 lead late in the first quarter before Boston scored 25 of the next 30 points.
Smart hit a 3 at the buzzer, giving Boston a 51-40 halftime edge.
76ers: Coach Brett Brown talked about how his team was dealing with injuries for the opener. "Going into tonight's game with nine and a half players," he said. "It's just part of the situation we've been dealt with." . G Tony Wroten (knee) and Kendall Marshall (knee) are both still sidelined after they each tore an ACL in January. C Joel Embiid will miss the season after the second surgery on his right foot.
Celtics: F Kelly Olynyk sat out his one-game suspension for his role in Cleveland F Kevin Love's left shoulder injury in last season's playoffs. . First-round draft pick R.J. Hunter was out sick and F/G James Young was inactive.
76ers: Host Utah in their home opener on Friday.
Celtics: Host Toronto on Friday.