| Name | Country | Years | Races | Wins | Championships |
|---|---|---|---|---|---|
| Officine Specializate Costruzione Auto | Italy | 1951,1953,1959 | 3 | 0 | 0 |
| Olsen | 1950 | 1 | 0 | 0 | |
| Onyx | Great Britain | 1989-1990 | 17 | 0 | 0 |
| Osella | Italy | 1980-1992 | 148 | 0 | 0 |
| Oswald Karch | 1953 | 1 | 0 | 0 | |
| Otello Nucci | 1962-1963,1965 | 3 | 0 | 0 | |
| Ottorino Volonterio | 1956-1957,1959 | 3 | 0 | 0 |
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