Less than five minutes into the 1h45m session, the red flags were waved after Callum Ilott hit the apex kerbs at Turn 5 before finding the tyre wall in his Juncos Hollinger Racing entry.
Barely three minutes after the session restarted, Rinus VeeKay pulled a wheelie over the same kerb, meeting the same fate in the wall.
When action resumed, it was Scott McLaughlin who first flirted with the 67-second barrier, his Chevrolet-powered Team Penske car lapping in 1m07.0182s.
While team-mate Josef Newgarden was his initial closest challenger, Scott Dixon of Chip Ganassi Racing made it a Kiwi 1-2, his Honda-powered car just 0.0775s off the top.
With 20 minutes to go, O’Ward, Friday’s pacesetter, was very close, less than a tenth further back, while Kyle Kirkwood led the Andretti Autosport squad in fourth, then went fastest.
However, he was quickly displaced by team-mate Colton Herta, by 0.0775s, who produced a 1m06.6868s, 106.240mph on the 1.968-mile street course.
Felix Rosenqvist got the second Arrow McLaren up to third, before Team Penske’s Will Power slotted ahead, 0.1275 off Herta before O’Ward then lowered the target to 1m06.5273s.
Kyle Kirkwood, Andretti Autosport Honda
Photo by: Jake Galstad / Motorsport Images
Newgarden and McLaughlin took turns at knocking him off his pedestal, moving up to second and third respectively.
In the closing 10 minutes, drivers started coming out on Firestone’s green sidewalled alternate compound tyres, with Dixon hitting the top spot with a 1m06.2407s.
However, this was never going to stand, and O’Ward slammed in a 1m05.9982s, which left him just 0.0167s ahead of Kirkwood, who got a nudge around the hairpin by a frustrated Power.
The reigning champion had already smacked a wall broadside, and he wasn’t the only Penske driver to make an error in the closing stages as McLaughlin suffered a spin.
That left Grosjean in third ahead of Herta – who was held up by McLaughlin’s incident – making it three Andretti cars in the top four.
McLaughlin retained fifth ahead of Rosenqvist, while the third Arrow McLaren of Alexander Rossi slithered with brakes locked down the Turn 9 escape road.
Dixon, Ericsson and Alex Palou were in seventh, eighth and ninth for Ganassi ahead of Newgarden, who rounded off the top 10.
Qualifying begins at 8.05pm BST.