Mike McCall Jr. had 10 points and seven rebounds for the Billikens (23-6, 11-4 Atlantic 10), who moved a half-game behind first-place Temple in the league standings. The Owls, however, hold the tiebreaker, a 72-67 victory against St. Louis in January.
The Billikens clinched a top-four seed and bye in next week's Atlantic 10 tournament.
Xavier led 39-29 at halftime but shot just 28 percent (7 for 25) afterward. That helped St. Louis take control with a 24-2 run over an 8:34 second-half stretch.
Conklin had 11 points and McCall hit two 3-pointers during the surge, which gave the Billikens a 53-42 lead with 9:44 remaining. The Musketeers got no closer than seven points the rest of the way.
Former Kentucky guard Ed Davender, the only Wildcat with at least 1,500 points and 400 assists, has died. He was 49.
Guard Naz Mitrou-Long, whose 2015-16 season ended in December due to hip problems, has been granted a hardship waiver for the coming season, Iowa State said.
As the Sooners look to life without superstar Buddy Hield and his sidekicks, it's clear their unseasoned players will need to grow up fast.