Cathryn Sophia has claimed an emphatic victory in the Kentucky Oaks at Churchill Downs.

After running third last month in the Ashland at Keeneland, she got a good start along with favourite Rachel's Valentina in the $US1 million ($A1.36 million) showcase for three-year-old fillies.

Cathryn Sophia found her opening at the top of the straight, overtook the weakening Lewis Bay and pulled away to beat Land Over Sea by 2-3/4 lengths.

It was a first Oaks win for her jockey Javier Castellano and John Servis who trained 2004 Kentucky Derby and Preakness champion Smarty Jones.

The Oaks was robbed of some of its glamour with the absence of unbeaten Songbird who is being treated for a fever.

Even though Cathryn Sophia was coming off a solid third in the Ashland behind Weep No More and Rachel's Valentina, Servis conceded he probably wouldn't have run her in the Oaks if Songbird was there.

The favourite Rachel's Valentina, who entered with consecutive runner-up finishes to Songbird in the Breeders' Cup Juvenile Fillies and Weep No More in the Ashland.

Although she was in the hunt for a while, Rachel's Valentina eventually finished sixth, with Weep No More seventh.