O’Neill, who outkicked Healy for gold in a thrilling 800m final at the National Championships last month, took over 1.5 seconds off her previous best, moving joint third on the Irish all-time list, with Healy moving fifth and taking over half a second off her previous PB.
The race was won by Britain’s Revee Walcott-Nolan in 1:59.05.
For Healy, it proved a positive tune-up ahead of her next outing at the Diamond League final in Zurich over 1500m next week, and it improves on the season’s best of 2:00.87 that she ran in a mixed race back in June.
Nick Griggs was in action over 1500m at the same meeting, the Tyrone athlete trying to book his spot at the World Championships in Tokyo.
He ran well to finish third in a season’s best of 3:35.25, which was short of the automatic qualifying standard of 3:33.00, with Adam Fogg winning in 3:34.11.
Griggs looks set to fall short of qualification for Tokyo in the 5000m when the deadline arrives this Sunday despite his Irish Under-23 record of 13:05.75 earlier this month in Belgium, and he will need to find another high-level race in the coming days if he is to have a shot over 1500m.
Brian Fay, who’s currently within the qualification quota for Tokyo over 5000m, was in action at the Lausanne Diamond League in Switzerland, where the almost relentless driving rain made fast times unlikely.
It proved a night to forget for the Dubliner, who was the last athlete to finish, clocking 13:36.61 for 18th.
The race was won by Belgium’s Isaac Kimeli who sprang a surprise on US star Grant Fisher, unleashing a huge kick to win in 13:07.67.
Olympic champion Noah Lyles suffered a notable defeat in the men’s 100m, the US star left trailing by Jamaica’s Oblique Seville, who ran 9.87 to Lyles’ 10.02. “I just had a horrible reaction to the gun,” said Lyles.
“That was the only thing wrong. Technically I felt good, my warm-up was good, but once you miss the start at this level the race is basically over. Physically I feel great, and I am confident every race will get better and better.”
Britain’s Keely Hodgkinson continued her superb comeback after a lengthy layoff with another victory, the Olympic champion clocking a meeting record of 1:55.69 to win the 800m ahead of Switzerland’s Audrey Werro.
Andrew Coscoran (1500m) and Shauna Bocquet (para 400m) are the sole Irish athletes due to compete in Brussels on Friday night, the penultimate meeting of the Diamond League season.
Meanwhile, Fionnuala McCormack has been announced as part of the line-up for the New York City Marathon in November.
The five-time Olympian is also due to race the marathon at next month’s World Championships in Tokyo, with Hiko Tonosa and Peter Lynch going for Ireland in the men’s race.