Complex sentence: The Oak Leaf Stakes is a race for thoroughbred horses restricted to two-year-old fillies and thus is a major prep race for the Breeders' Cup Juvenile Fillies.
Split into simpler sentences: The Oak Leaf Stakes is a race for thoroughbred horses. The race is restricted to two-year-old fillies and thus is a major prep race for the Breeders' Cup Juvenile Fillies.