| Number of Mares Reported Bred to Thoroughbred Stallions in 2021 | |||||
| A B C D E F G H I J K L M N O P Q R S T U V W X Y Z | |||||
| Stallion | Mares Bred | State | |||
| O'Prado Again | 4 | AB | |||
| Ocean Knight | 17 | LA | |||
| Oceanus | 1 | AL | |||
| Oh So Regal | 5 | NM | |||
| Old Forester | 20 | ON | |||
| Old Puerto | 1 | ND | |||
| Om | 78 | CA | |||
| Omaha Beach | 215 | KY | |||
| One Go All Go | 6 | NE | |||
| One Liner | 15 | LA | |||
| One Sided | 5 | OH | |||
| Oneniceday | 4 | AR | |||
| Optimizer | 4 | KY | |||
| Orbiting | 2 | CA | |||
| Ordained | 10 | AR | |||
| Oscar Nominated | 29 | CA | |||
| Oscar Performance | 80 | KY | |||
| Osiris of the Nile | 2 | CA | |||
| Our Celebration | 1 | AL | |||
| Our Entourage | 6 | WV | |||
| Outflanker | 1 | MD | |||
| Outwork | 146 | KY | |||
| Oxbow | 28 | KY | |||