| O A Anaya | Deauville 08-Apr-25, Cross The Tracks |
| O Bizoux | Enghien 12-Oct-23, Idee Meloc |
| O Briand | ParisLongchamp 14-Jul-25, Juldiko |
| O De | La Teste De Buch 11-Aug-25, Lordrecord |
| O Greenall & J Guerriero | Uttoxeter 17-Oct-25, Eagles Gold |
| O Hirata | Chukyo 17-Mar-24, Queen's Pleiades |
| O J Power | Cork 12-Jul-25, Tiffany Mae |
| O Jauregui | Del Mar 25-Jul-25, Dance Ella Dance |
| O Martinez | Turf Paradise 25-Apr-25, Big Jack |
| O McKiernan | Punchestown 15-Oct-25, Head For Home |
| O Murphy | Wetherby 15-Oct-25, Caelan |
| O Nicholls | Cheltenham 03-May-25, All Loved Up |
| O Picard | Le Mans 18-Dec-23, Iakim |
| O Refai | Meydan 15-Mar-25, Lenormand |
| O Regley | Les Sables 11-Oct-25, Hey Fabulous |
| O Schnakenberg | Le Croise-Laroche 07-Oct-25, Alatemagicbaby |
| O Sherwood | Uttoxeter 19-Jul-23, Jersey Wonder |
| O Signy | Chepstow 10-Oct-25, Block Rockin Beats |
| O Trigodet | Toulouse 11-Oct-25, Hong Kong Star |
| Oberdan Pereira | Gavea 23-Apr-25, Lilli Bel |
| Obidio Hernandez | Sam Houston Race Park 23-Apr-23, Cletajean |
| Octavio Saenz | Zia Park 04-Dec-23, Jl Juanita |
| Ogura Kensuke | Nagoya 03-Sep-25, Dio Spada |
| Oliver Cole | Chelmsford City 17-Oct-25, Arran |
| Oliverio L L | Club Hipico Santiago 22-Apr-25, Roce Secreto |
| Oliverio Martinez | Club Hipico Santiago 17-May-25, Capitana Marvel |
| Olivia Cairns | Ipswich 17-Oct-25, La Pittrice |
| Olivia M Duffy | Waverley 19-Oct-25, Ghazzah |
| Olivia Osborne | Wodonga 18-Oct-25, Dashing Donkey |
| Olivia Raeburn | Rockhampton 29-Dec-18, Craiglea Magnum |
| Ollie Matthews | Horseshoe Indianapolis 19-Jul-24, Encores Dynasty |
| Ollie Pears | Southwell 17-Oct-25, Heartened |
| Ollie Sangster | ParisLongchamp 19-Oct-25, Celestial Orbit |
| Olly Williams | Market Rasen 14-Oct-25, Lady Velouette |
| Omani Williams | Saratoga 23-Aug-25, King Of Fog |
| Omar A Cardoso | Palermo 19-May-23, Suno Top |
| Omar Beltrame | San Isidro 05-Dec-24, Que Dionisio |
| Omar Castro | San Isidro 01-Dec-24, Aladdins Cave |
| Omar Cruz | Turf Paradise 30-Apr-25, Queens Effort |
| Omar D Castro | La Plata 21-Sep-23, Claro Nistel |
| Omar F Labanca | Palermo 10-Nov-23, As Del Tango |
| Omar Farfan | San Isidro 26-Apr-25, Payaso Azul |
| Omar Guadarrama | Turf Paradise 06-Mar-25, Horns N Halos |
| Omar Ilarione | San Isidro 13-Feb-25, Gracioso Nak |
| Omar L Villarreta | Palermo 27-Jun-23, Vueltera Best |
| Omar Labanca | San Isidro 01-May-25, Sixties Girl |
| Omar P Tournoud | Palermo 31-Oct-23, Violenta Storm |
| Omar Quintilli | San Isidro 17-Oct-24, Al Mufti |
| Omar R Farfan | San Isidro 13-Mar-25, Melodia Seattle |
| Omar Tournoud | San Isidro 17-Apr-25, Daisy Rich |
| Omar Vargas | Sunray Park 24-May-25, Blur |
| Omar Villarreta | San Isidro 24-Oct-24, Manjar Exquisito |
| Orlando Fernandes | Gavea 29-Oct-24, Rack Sameah |
| Orlando Guerrero | Turf Paradise 23-Apr-25, Project Amazon |
| Orlando Noda | Belmont At The Big A 03-Oct-25, Tarpaulin |
| Orlando Perez | San Isidro 15-May-25, Flaming June |
| Orlando Rose | Delaware Park 10-Oct-25, Girvana |
| Orlando Vinardi | San Isidro 07-Sep-23, Buenos Anos |
| Orrin Cogburn | Remington Park 03-Oct-25, Texan Farmer |
| Ortega | San Isidro 31-Oct-24, Land Hopper |
| Ortis Henry | Finger Lakes 26-Nov-24, Watch Hill |
| Osamu Hirata | Niigata 19-Oct-25, Auxo |
| Osborne Smith | Charles Town 18-Oct-25, River Rewards Rvf |
| Oscar A Barrionuevo | Palermo 14-Nov-23, Nildita |
| Oscar A Farias | Palermo 27-Jun-23, Maxi Texano |
| Oscar A Galassi | Palermo 19-May-23, Burlata |
| Oscar A Galli | Palermo 06-Jun-23, Lucci |
| Oscar A Kalk | Palermo 30-May-23, Cebreiro |
| Oscar A Rebora | San Isidro 11-Jan-24, Grand Implosion |
| Oscar A Zavatarelli | Palermo 09-Jun-23, Flicka Fil |
| Oscar Almada | San Isidro 05-Apr-25, Mr. Chino |
| Oscar Arredondo | San Isidro 04-May-23, Hit Del Verano |
| Oscar Barrera III | Saratoga 26-Jul-25, Unbridled Bomber |
| Oscar Blanchet | San Isidro 15-Dec-24, Mi Amigo Cocho |
| Oscar Cagnoli | San Isidro 06-Jun-24, Quala De Gusti |
| Oscar Cancela | Palermo 15-Oct-23, Fix Felix |
| Oscar Carrete | Sunland Park 31-Mar-25, One Famous Molly |
| Oscar Cincunegui | San Isidro 15-May-25, Luca Changretta |
| Oscar Constancio | Sam Houston Race Park 30-Apr-23, Daring Goodtime |
| Oscar Conti | San Isidro 10-Apr-25, Russian Sindar |
| Oscar Corsi | San Isidro 09-Sep-23, Pequena Victoria |
| Oscar D Lopez | Palermo 26-Jun-23, Polirun |
| Oscar E Mihura | San Isidro 12-Oct-23, Julis Bomb |
| Oscar F Conti | Palermo 03-Jul-23, Million Reasons |
| Oscar F Fravega | Palermo 29-Oct-23, Inca Music |
| Oscar Flores | Remington Park 19-Oct-25, Charmonte |
| Oscar Fravega | San Isidro 11-Apr-24, Cuchcabal |
| Oscar G G | Hipodromo Chile 02-May-25, Don't Cry Daddy |
| Oscar G Villarroel | Palermo 27-Jun-23, A La Cumbre |
| Oscar Gonzalez | Gulfstream 27-Jun-25, Frisson |
| Oscar Heredia | Santa Anita 17-Jun-24, Anastasio's Secret |
| Oscar Hernandez | San Isidro 10-Aug-24, Lethal Candy |
| Oscar Ibarra | Palermo 24-Jun-24, Act Now |
| Oscar Kalk | Palermo 17-Oct-23, Lombada |
| Oscar M M | Valparaiso Sporting Club 14-Nov-23, Torito De Pucara |
| Oscar Mancilla | |
| Oscar Mihura | San Isidro 03-Oct-24, Gran Luchador |
| Oscar Modica | Fair Grounds 09-Feb-24, Exacta |
| Oscar Nis | San Isidro 05-Apr-25, Bearnes |
| Oscar Preciado | Horseshoe Indianapolis 09-Oct-25, Ap Miss Lunas Tide |