The old adage rings true—somebody's "O" has gotta go. See dates previously undefeated boxers were defeated for the first time in their professional careers and by whom.
| Date "O" Went | Boxer | Defeated By |
|---|---|---|
| April 19, 2008 | Chris Henry | Adrian Diaconu |
| April 12, 2008 | James McGirt Jr | Carlos De Leon Jr |
| March 22, 2008 | Michael Katsidis | Joel Casamayor |
| March 21, 2008 | Andy Lee | Brian Vera |
| March 8, 2008 | Juan Diaz | Nate Campbell |
| February 23, 2008 | Sultan Ibragimov | Wladimir Klitschko |
| February 9, 2008 | Paul Williams | Carlos Quintana |
| January 26, 2008 | Eddie Chambers | Alexander Povetkin |
| December 8, 2007 | Ricky Hatton | Floyd Mayweather Jr |
| November 3, 2007 | Mikkel Kessler | Joe Calzaghe |
| September 29, 2007 | Jermain Taylor | Kelly Pavlik |
| August 11, 2007 | Rey Bautista | Daniel Ponce de Leon |
| July 7, 2007 | Vic Darchinyan | Nonito Donaire |
| April 14, 2007 | Nikolay Valuev | Ruslan Chagaev |
| March 3, 2007 | Allan Green | Edison Miranda |
| December 16, 2006 | Jason Litzau | Jose Andres Hernandez |
| December 2, 2006 | Carlos Quintana | Miguel Angel Cotto |
| November 11, 2006 | Calvin Brock | Wladimir Klitschko |
| September 23, 2006 | Edison Miranda | Arthur Abraham |
| August 10, 2006 | Brian Viloria | Omar Nino Romero |
| June 10, 2006 | Paul Malignaggi | Miguel Angel Cotto |
| March 4, 2006 | Jeff Lacy | Joe Calzaghe |
| December 2, 2005 | Robert Guerrero | Gamaliel Diaz |
| September 24, 2005 | Samuel Peter | Wladimir Klitschko |
| August 20, 2005 | Rocky Juarez | Humberto Soto |
| August 5, 2005 | Sechew Powell | Kassim Ouma |
| May 5, 2005 | Cornelius Bundrage | Sechew Powell |
| April 23, 2005 | Kermit Cintron | Antonio Margarito |
| February 17, 2005 | Daniel Ponce de Leon | Celestino Caballero |
| September 18, 2004 | Joey Gilbert | Peter Manfredo Jr |
| August 18, 2004 | Peter Manfredo Jr | Alfonso Gomez |
| June 27, 2004 | Holly Holm | Rita Turrisi |
| May 8, 2004 | Vonda Ward | Ann Wolfe |
| July 12, 2002 | Roman Karmazin | Javier Castillejo |
| June 22, 2002 | Erik Morales | Marco Antonio Barrera |
| June 1, 2002 | Serguei Lyakhovich | Maurice Harris |
| April 13, 2002 | Luis Collazo | Edwin Cassiani |
| January 26, 2002 | Shane Mosley | Vernon Forrest |
| January 12, 2002 | Joel Casamayor | Acelino Freitas |
| November 3, 2001 | Zab Judah | Kostya Tszyu |
| September 29, 2001 | Felix "Tito" Trinidad | Bernard Hopkins |
| June 15, 2001 | Alfonso Gomez | Ishe Smith |
| January 20, 2001 | Diego Corrales | Floyd Mayweather Jr |
| December 2, 2000 | Fernando Vargas | Felix Trinidad |
| June 24, 2000 | Junior Witter | Zab Judah |
| June 23, 2000 | Antonio Tarver | Eric Harding |
| September 18, 1999 | Oscar De La Hoya | Felix Trinidad |
| February 13, 1999 | Ike Quartey | Oscar De La Hoya |
| December 19, 1998 | Hasim Rahman | David Tua |
| December 5, 1998 | Wladimir Klitschko | Ross Puritty |
| March 21, 1997 | Roy Jones Jr | Montell Griffin |
| November 22, 1996 | Marco Antonio Barrera | Junior Jones |
| March 15, 1996 | Shannon Briggs | Darroll Wilson |
| February 9, 1996 | Manny Pacquiao | Rustico Torrecampo |
| November 18, 1994 | James Toney | Roy Jones Jr |
| October 17, 1994 | Antonio Margarito | Victor Lozoya |
| August 21, 1994 | Ronald "Winky" Wright | Julio Cesar Vasquez |
| November 17, 1992 | Arturo Gatti | King Solomon |
| November 13, 1992 | Evander Holyfield | Riddick Bowe |
Also see boxing rankings.