Guard Cameron Clark and forward Romero Osby led OU with 14 points, and guard Steven Pledger added 11, but it wasn't enough in the Sooners' effort to win three straight games, including two as underdogs, en route to the tournament title.
That's because forward Brian Conklin for the Billikens stood in their way, scoring 25 points on an impressive 9-of-11 shooting night.
But even though he and Saint Louis eventually stormed away in the end behind his strong performance, it was close for a good portion of the contest.
Teams traded the lead four times in a first half that featured five ties.
OU led its first time at 20-19 after guard Carl Blair canned a three with 7:39 remaining in the opening half.
The Billikens regained the lead just 36 seconds later on their next score.
The Sooners were on top for the second and final time at 29-27 when guard Calvin Newell busted a jumper inside of four minutes to go in the first half.
From that point, it was basically all Saint Louis.
They used a 12-2 run to transform a two-point deficit into an eight-point halftime advantage at 39-31.
It was never closer than eight again from that point on, as the Billikens led by as much as 21 in the final minute.
The Sooners, who rank second nationally in rebounding, actually out-boarded the Billikens 31-29 but shot just 41.7 percent to 60.4 percent for their counterparts.
With the loss, OU falls to 4-1 on the season, and it returns to action against Sacramento State at 7 p.m. Friday inside Lloyd Noble Center.