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The 2014 European 100m champion held off the fast-finishing James Ellington and his friend and rival CJ Ujah in a showdown that lived up to expectations.
Earlier his training partner Asha Philip won her third British 100m title to join him in the GB team for Rio.
Tiffany Porter beat her younger sister Cindy Ofili in the 100m hurdles.
The 28-year-old Dasaolu got out of his blocks well as Ellington was left, and held a metre lead at halfway.
With Richard Kilty and Harry Aikines-Aryeetey struggling, and veteran Dwain Chambers never in the picture, Ujah seemed the only threat to the man who ran 9.91 seconds at these championships three summers ago.
But Ellington came through late outside them to steal both second place and the second automatic Olympic place.
A tailwind of three metres per second added a gloss to the times - 9.93 secs for Dasaolu, 9.96 for Ellington and 9.97 for Ujah - but with Britain's 200m men also in form, the GB sprint relay squad appears in excellent shape going to Brazil.
"Seven guys on the line had the [Olympic qualifying] standard, so I knew that if I didn't finish top two, top three, my dream of going to Rio in the 100m was in jeopardy," said Dasaolu.
"Top three under 10 seconds in the same race bodes well for the relay and for sprinting as a whole. Jamaica and America are out there, but if they make any mistakes we'll be nipping at their heels and ready to go for the gold."
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In the absence of British record holder Dina Asher-Smith - focusing on the longer sprint this weekend - from the women's 100m, Philip upset pre-race favourite Desiree Henry as she dominated in 11.17 secs, with 19-year-old talent Daryll Neita taking second in 11.24 and Henry in third.
Andy Butchart and Tom Farrell both sealed Olympic places in the 5,000m, while Sophie Hitchon confirmed her place in the hammer with a characteristically dominant throw and Holly Bradshaw did the same in the pole vault with a season's best clearance of 4.60m.
James Dasaolu ran a wind-assisted 9.93 seconds to win a thrilling battle of the British sprint kings and secure his place at his second Olympic Games.