Winfrey brought on Oz as a regular guest on The Oprah Winfrey Show for more than five seasons in the early 2000s, giving him his big breakthrough.
Oz led McCormick by less than 1,000 votes as state officials conducted a recount after the May 17 primary.
A likely recount would mean that the race between Dr. Mehmet Oz and David McCormick might not be decided until June 8, the deadline for counties to report their results to the state.