O'Connor failed to reach a single final on her first appearance at the championships last year however she qualified for the 200m medley medal showdown on the first day in France.
The 16-year-old suffered heartache as she finished fourth but there was to be no stopping her in her second final as she clocked a new personal best on the way to bronze in the 100m medley.
O'Connor touched home in 59.72seconds for third, 0.89 behind Hungarian Katinka Hosszu who claimed yet another gold in Chartres while Zsuzsanna Jakabos finished in between the two in second.
And, while there was delight for O'Connor, there was heartache for her British teammate Sophie Smith as she finished outside the medal positions in fifth after clocking 1:00.54minutes.
There was similar anguish for both Hannah Miley and Aimee Willmott in the 400m freestyle final that saw Olympic champion Muffat break Brit Jo Jackson's world record.
Miley swam a Scottish record of 4:00.39 however that was to be just over half a second shy of what was needed for her to claim a third medal in Chartres as she finished fourth.
Willmott, who took 800m freestyle bronze on day two, was a place back in fifth while Muffat lowered Jackson's record by 0.07 to 3:54.85 for gold with Lotte Friis second and Coralie Balmy third.
Another Brit Lauren Quigley also missed out on a medal in the 50m backstroke, qualifying for the final as the joint-third fastest before finishing sixth in the medal showdown.
Quigley did not disgrace herself though, rather the opposite, clocking a personal best of 27.08 in the final that was only 0.11 off Simona Baumrtova of the Czech Republic who took bronze.
Laure Manaudou delighted the home crowd by taking gold in 26.78 with Croatia's world record holder over the distance Sanja Jovanovic 0.06 behind in second.
Marco Loughran failed to reach the 100m backstroke final, finishing one place shy of doing so, while Smith was 15th in the 100m butterfly semi-finals moments after her 100m medley effort.
Meanwhile Italy's Gregorio Paltrinieri won the men's 1500m freestyle and Hungarian Laszlo Cseh claimed his third gold medal of the championships in the 200m butterfly.
Russian enjoyed a one-two in the men's 100m freestyle final as Vladimir Morozov took gold and Evgeny Lagunov silver with France's Yannick Agnel in third.
In the men's 50m breaststroke Fabio Scozzoli collected his second medal in Chartres by just 0.02over Aleksander Hetland while France won the mixed 4x50m freestyle relay.