Hours after the league was rocked by a massive illegal gambling probe, reigning NBA Most Valuable Player Gilgeous-Alexander dragged the Thunder to victory.
The Canadian added eight rebounds and five assists while hitting 15-of-31 shots from the floor and 23-of-26 free throws to lead a 45-of-51 Thunder effort from the line.
The 27-year-old guard, last season's top NBA scorer, netted nine points in the second overtime to lift Oklahoma City over a determined Pacers squad.
After the Thunder won their season opener at home Tuesday over Houston 125-124 in double overtime, they became the first club in NBA history to go into double overtime in their first two games of a campaign.
Gilgeous-Alexander, whose prior career-best was 54 points, managed his fifth NBA career 50-point game.
Ajay Mitchell came off the bench to score 26 points for Oklahoma City and Aaron Wiggins added 23 points, while Isaiah Hartenstein grabbed 14 rebounds and Chet Holmgren contributed 15 points and 12 rebounds.
Gilgeous-Alexander's three-point play gave the Thunder a 127-126 lead in the second overtime and his bank shot with 2:47 to play in the second extra period gave the reigning champions a 132-128 edge.
The Pacers never came within four again as Gilgeous-Alexander added four free throws in the final 30 seconds.
Canadian guard Bennedict Mathurin led Indiana with 36 points and grabbed 11 rebounds while Cameroonian playmaker Pascal Siakam added 32 points and 15 rebounds for the Pacers.
Warriors beat Denver
In the night's only other game, Stephen Curry scored 42 points to lead host Golden State over Denver 137-131 in overtime as the Warriors also started 2-0.
"Just trying to be decisive, to get to my spots," Curry said. "We were as disciplined as you can imagine. Great win for us. Sense of urgency to get off to a good start."
Denver's Aaron Gordon scored a career-high 50 points, going 10-of-11 from three-point range, and three-time NBA MVP Nikola Jokic added 21 points, 13 rebounds and 10 assists for the Nuggets.
The Thunder visited Indianapolis to face a Pacers squad far different than the one they played for the crown last June.
Indiana were without star guard Tyrese Haliburton, who suffered a torn Achilles tendon in game seven of last season's finals and is out for the season, and center Myles Turner, who departed for Milwaukee.
There was more injury misery for Indiana as guard Andrew Nembhard was lost for the rest of the game to a left shoulder injury in the second quarter.
A Gilgeous-Alexander free throw with 13 seconds remaining in regulation gave the Thunder a 113-111 advantage but Siakam sank a 21-foot jumper to send the game into overtime.
Gilgeous-Alexander and Mathurin traded hoops in the last 30 seconds of overtime to force another extra period.