, the 2010 British Open champion.
Four-time major winner McIlroy, playing his first major round since becoming a father with the birth of daughter Poppy two weeks ago, made three of his four birdies on par-3 holes.
"Got off to a really nice start. Hit the ball very well, kept it in play," McIlroy said. "I sort of walked off the green a little disappointed but if someone had given me a 67 at the start of the day I'd have taken it."
Westwood, who would be the oldest US Open winner, sank a 12-foot birdie putt at the ninth hole, his last of the day, to match McIlroy.
"It was a good round of golf. Really solid in every aspect of the game, so I'm pleased with that start," said Westwood, who has 12 top-five major finishes without a victory, three of them in US Opens.
Oosthuizen holed out from 148 yards in the second fairway for an eagle and closed with a birdie at the ninth from inside four feet.
Tiger, DJ, Phil stumble
Top-ranked Dustin Johnson and 15-time major winner Tiger Woods each opened on 73.
Johnson, who won the PGA Tour Championship two weeks ago, managed only two birdies against a double bogey and three bogeys.
"It wasn't too bad. Just didn't drive it in the fairway enough," Johnson said. "I rolled it really well, but I just didn't make any putts."
Woods, chasing a fourth US Open title at age 44, grinded all day but closed bogey-double bogey for a 73.
Woods birdied three in a row from the ninth through 11th holes but made three bogeys and a double in the last six holes.
"I needed to finish off the round better," Woods said. And I didn't."
Five-time major winner Phil Mickelson shot 79, the 50-year-old left-hander opening birdie-birdie but following with nine bogeys and a double bogey.
"It's a disappointing day," Mickelson said. "I drove it poorly and I putted poorly. I just played terrible."
First-round scores on Thursday in the 120th US Open Golf Championship at Winged Foot in Mamaroneck, New York:
65 - Justin Thomas (USA)
66 - Patrick Reed (USA), Matthew Wolff (USA), Thomas Pieters (BEL)
67 - Louis Oosthuizen (RSA), Lee Westwood (ENG), Rory McIlroy (NIR)
68 - Joaquin Niemann (CHI), Rafael Cabrera Bello (ESP), Harris English (USA), Xander Schauffele (USA), Jason Kokrak (USA), Brendon Todd (USA)
69 - Davis Thompson (USA), John Pak (USA), Rickie Fowler (USA), Bryson DeChambeau (USA), Jon Rahm (ESP), Shaun Norris (RSA), Rory Sabbatini (SVK)
70 - Matt Wallace (ENG), Erik van Rooyen (RSA), Christiaan Bezuidenhout (RSA), Patrick Cantlay (USA), Kurt Kitayama (USA), Chun An Yu (TPE), Michael Thompson (USA), Im Sung-jae (KOR), Zach Johnson (USA), Will Zalatoris (USA), Danny Lee (NZL)
71 - J.T. Poston (USA), Andy Sullivan (ENG), Martin Kaymer (GER), Thomas Detry (GER), Hideki Matsuyama (JPN), Webb Simpson (USA), Adam Scott (AUS), Lucas Glover (USA), Corey Conners (CAN), Sebastian Munoz (COL), Chan Kim +(USA), Taylor Pendrith (CAN), Renato Paratore (ITA), Adam Long (USA), Cameron Smith (AUS), Andy Ogletree (USA), Abraham Ancer (MEX), Viktor Hovland (NOR), Romain Langasque (FRA), Shugo Imahira (JPN), An Byeong-hun (KOR), Lanto Griffin (USA), Stephan Jaeger (GER), Adrian Otaegui (ESP)
72 - Preston Summerhays (USA), Jimmy Walker (USA), Paul Waring (ENG), Jason Day (AUS), Ryo Ishikawa (JPN), Adam Hadwin (CAN), Mackenzie Hughes (CAN), Troy Merritt (USA), Lucas Herbert (AUS), Kim Si-woo (KOR), Bubba Watson (USA), Billy Horschel (USA), Takumi Kanaya (JPN), Alex Noren (SWE)
73 - Daniel Balin (USA), Joel Dahmen (USA), Ricky Castillo (USA), Kevin Streelman (USA), Jazz Janewattananond (THA), Tyler Duncan (USA), Jordan Spieth (USA), Tiger Woods (USA), Justin Rose (ENG), Charles Howell (USA), Richy Werenski (USA), Mike Lorenzo-Vera (FRA), Jim Herman (USA), Andrew Putnam USA), Chesson Hadley (USA), Ryan Palmer (USA), Bernd Wiesberger (AUT), Marc Leishman (AUS), Cameron Champ (USA), Daniel Berger (USA), Dustin Johnson (USA)
74 - Brandon Wu (USA), Ryan Fox (NZL), JC Ritchie (RSA), Brian Harman (USA), Kang Sung (KOR), Tom Lewis (ENG), John Augenstein (USA), Tyrrell Hatton (ENG), Henrik Stenson (SWE), Sergio Garcia (ESP), Matt Kuchar (USA), Steve Stricker (USA), Robert MacIntyre (SCO), Scott Hend (AUS), Gary Woodland, Matt Fitzpatrick (ENG), Tommy Fleetwood (ENG)
75 - Curtis Luck (AUS), Greyson Sigg (USA), Chez Reavie (USA), Kevin Na (USA), Ian Poulter (ENG), Sandy Scott (SCO), Branden Grace (RSA), Keegan Bradley (USA), Brandt Snedeker (USA)
76 - Victor Perez (FRA), Collin Morikawa (USA), Graeme McDowell (NIR), Dan McCarthy (USA), Eduard Rousaud (ESP), Matt Jones (AUS), Shane Lowry (IRL), Kevin Kisner (USA), Paul Casey (ENG), Mark Hubbard (USA), Lee Hodges (USA)
77 - Rasmus Hojgaard (DEN), Danny Willett (ENG), Justin Harding (RSA), Matthias Schwab (AUT), Cole Hammer (USA)
78 - Max Homa (USA), Eddie Pepperell (ENG), Sami Valimaki (FIN), Ryan Vermeer (USA), James Sugrue (IRL)
79 - Phil Mickelson, Davis Riley (USA)
Lukas Michel (AUS)