The Thunder, trying to become the first team since Golden State in 2017 and 2018 to repeat as champions, routed the Phoenix Suns 119-84.
The Celtics, seeded second in the Eastern Conference behind Detroit, dominated the Philadelphia 76ers 123-91, but the Pistons trailed all the way in a 112-101 loss to the Magic.
And San Antonio star Victor Wembanyama made a triumphant playoff debut, scoring 35 points as the Spurs beat the Portland Trail Blazers 111-98.
Shai Gilgeous-Alexander, the reigning NBA Most Valuable Player and Finals MVP, scored 25 points, connecting on just five of 18 from the field but drilling 15 of his 17 free throw attempts.
He added seven assists and two blocked shots before sitting out the entire fourth quarter.
Jalen Williams added 22 points and Chet Homgren scored 16 for the Thunder, who are trying to become the first team since the Golden State Warriors in 2017 and 2018 to repeat as champions.
They wasted no time in seizing control in front of a revved crowd at the Paycom Center, leading by as many as 25 on the way to a 21-point halftime lead and by as many as 39 in the fourth quarter.
Devin Booker scored 23 points for Phoenix, but the Suns, who scraped through the play-in to claim the Western Conference eighth seed, were largely helpless against a Thunder team that amassed the best record in the league with 64 regular-season wins.
"It was a great defensive performance," Thunder coach Mark Daigneault said. "I thought the guys were really competitive, played on a string, great communication, covered for each other. (It) wasn't perfect, but we had each other's backs.
"We forced them to earn everything."
While the well-rested Thunder came out firing, the Pistons looked rusty after having a week off and the Magic, who beat Charlotte on Friday to secure their playoff spot, took full advantage.
Paolo Banchero scored 23 points and Franz Wagner scored 11 of his 19 in the fourth quarter as the Magic withstood a 39-point performance from Pistons star Cade Cunningham.
Detroit trailed by as many as 13 in the first quarter and by 11 early in the third. A Cunningham three-pointer tied it with 7:04 left in the third, but the Magic pulled away again.
"I liked our composure the whole game, our energy," Wagner said after the Magic handed the Pistons an 11th straight home playoff defeat, a skid dating back to 2008.
'Incredible feeling'
In Boston, Jayson Tatum and Jaylen Brown combined for 51 points as the Celtics led wire to wire against the Sixers, who have lost their last six post-season series against Boston.
The indomitable duo took full advantage of the Sixers' dire shooting struggles to secure the Celtics' largest-ever margin of victory in a playoff opener.
Tatum led the charge, racking up 21 points in the first half. It was Brown's turn to go on the rampage after the break, pouring in 16 points in the third quarter alone to take the game beyond Philadelphia's grasp.
Both men sat out most of the final quarter, resting up for Tuesday's second home game before the best-of-seven series moves to Philadelphia, but the Celtics' lead continued to grow in a wire-to-wire win.
In one of the NBA's classic rivalries, Boston have won their last six postseason series against the Sixers - a streak that goes back to the 1985 Eastern Conference finals.
Tatum showed no ill effects from the torn Achilles tendon he suffered almost a year ago, exhibiting why his return has made the surging Celtics many pundits' pick for this year's NBA title.
He finished with 25 points, 11 rebounds and seven assists.
"It's an incredible feeling. You know, not too long ago, I wasn't even sure if I was going to be able to play this season - let alone get an opportunity to play in the playoffs," said Tatum.
"Today was probably the most excited and, like, relaxed and grateful I've been in... my nine years of being in the playoffs."
In San Antonio, the second-seeded Spurs led by as many as 16 points on the way to a 59-49 half-time lead.
Portland, led by 30 points and 10 rebounds from Deni Avdija, cut the deficit to two early in the third quarter.
But the Spurs pulled away again, thrilling the San Antonio fans who have waited since 2019 to see the five-time champion franchise return to the playoffs.
"We were solid, but job's not done," said Wembanyama, who earlier in the day joined Gilgeous-Alexander and Denver's Nikola Jokic as a finalist for regular-season MVP honors.
