The Friars (21-12) advance to face second-seeded and No. 2-ranked Villanova (29-4), 87-68 winners over Butler, in the Big East title game on Saturday night.
Kyron Cartwright added 15 points and six assists and Nate Watson scored 14 for Providence, which is making its first Big East title game appearance since 2014 when it defeated Creighton to win its second conference tourney crown.
Kerem Kanter had a game-high 18 points, Paul Scruggs had 15 and Trevon Bluiett 13 for Xavier (28-5), which had won 13 of its final 14 games to claim the Big East's regular-season title.
Diallo's jumper with 2:25 left in OT put the Friars up 71-70, their first lead since the 17:13 mark of the opening half. Cartwright's jumper with 57 seconds left stretched the Providence lead to three.
Two Bluiett free throws with 40 ticks on the clock brought the Musketeers within a point. Providence's Isaiah Jackson missed a layup with 12 seconds left following a timeout, but Xavier's J.P Macura turned the ball over with just seven seconds left on the clock.
Diallo's free throws with six seconds left put the Friars back up three.
Kanter hit what appeared to be a tying 3-pointer as time expired, but the shot came after the final buzzer.
Providence was likely NCAA Tournament bound already after Thursday's 72-68 overtime win against fourth-seeded Creighton. The Friars are a No. 10 seed in ESPN's latest tourney projections.
The Friars and Musketeers split their two regular-season meetings, with Providence winning 81-72 at home on Jan. 6 and Xavier prevailing 84-74 on its home court Feb. 28.
Xavier led 43-29 at halftime after leading by as many as 16 points in the first period.
The Musketeers took their biggest lead, 52-35, on Macura's 3-pointer with 15:20 remaining in the second. Providence's furious 29-12 spurt tied the game at 64 on Bullock's jumper with 3:46 left.
Bluiett and Bullock traded a pair of free throws to knot the score at 66 heading into the final minute. Xavier's Sean O'Mara hit a go-ahead jumper with 35 seconds left, but Providence's Cartwright tied it again with two free throws with 12 ticks left.
Bluiett missed a game-winning layup at the buzzer, forcing overtime.
--Field Level Media
Providence | Xavier | |
Rodney Bullock 17 | Scoring | Kerem Kanter 18 |
Kyron Cartwright 6 | Assists | Trevon Bluiett 3 |
Isaiah Jackson 8 | Rebounds | Naji Marshall 12 |
Alpha Diallo 7 | Free Throws Made | Trevon Bluiett 8 |
Rodney Bullock 2 | Steals | Kaiser Gates 2 |
Rodney Bullock 1 | Blocks | J.P. Macura 1 |
Team | Points | FG% | 3PM-3PA | FTM-FTA | Assists | Rebounds | Blocks | Steals | Turnovers |
Providence | 75 | 43.8 | 3-19 | 16-20 | 13 | 33 | 4 | 4 | 7 |
Xavier | 72 | 36.8 | 5-22 | 17-18 | 12 | 41 | 2 | 6 | 10 |