Root runs riot with half-century for England in fifth Ashes Test
England, in a match they have to win to square the series at 2-2, were 265-4 in their second innings at tea on the third day - an imposing lead of 253 runs.
Root, in sublime touch, was 61 not out off 58 balls after sharing a stand of 73 for the third wicket with England captain Ben Stokes (42).
Stokes and Harry Brook departed in quick succession to leave the home side 222-4 but Jonny Bairstow kept up the attack as he joined Yorkshire team-mate Root in the middle, with an unbeaten 34 off 35 balls.
England's lead was already significant given there have only been five successful chases of more than 200 runs to win in the fourth innings during 143 years of Test cricket at The Oval.
Zak Crawley, the leading run-scorer in the series, launched England's innings in style to take the hosts to 130-1 at lunch.
Stokes hooked Josh Hazlewood for six in the first over after lunch, with Mitchell Starc dropping a catch on the fine leg boundary as he tried to stay inside the rope.
Crawley added just two runs to his lunch score of 71 when, going for another off-drive, he edged Australia captain Pat Cummins to Steve Smith at second slip, with England now 140-2.
Root charge
Root then demonstrated the range of his strokeplay by reverse-scooping Mitchell Marsh for a superbly timed six and next ball gliding the all-rounder down the legside for four.
The former England captain then drove Starc down the ground for four, with even his mishits going for boundaries - next ball an under-edged cut off the left-arm quick bounced over the head of wicketkeeper Alex Carey.
But there was nothing lucky about Root's third four in as many balls off Starc, a deliberate uppercut over the slips.
The sunny overhead conditions, with the ball not swinging in the air or deviating much off the pitch, were ideal for attacking batting.
Stokes, however, gave his wicket away when caught at mid-on off a mistimed drive against spinner Todd Murphy.
England needed only one over to erase their 12-run first-innings deficit at the start of play on Saturday.
Crawley, who had struck a cover drive for four off the first ball of the series, repeated the shot off the first ball on Saturday in a clear sign of England's intentions before left-hander Ben Duckett ended the over with successive boundaries.
But a stand of 79 ended when Duckett, on 42, edged a full-length Starc delivery to wicketkeeper Alex Carey.
Come tea, however, that was the only wicket Starc had taken in an expensive return of 1-64 in 10 overs.
With Moeen Ali ruled out of batting in his number three position after being off the field all day Friday with a groin injury, Stokes promoted himself up the order.
Australia, as the holders, have already retained the Ashes at 2-1 up.
But they must avoid defeat at The Oval if they are to win their first Ashes series in England since 2001.