The Pennsylvania seat has for months been seen as the most likely pickup opportunity for Democrats in the evenly divided Senate.
Polls show a close race between Fetterman and Oz, a celebrity heart surgeon who is endorsed by former President Donald Trump.
In a sign of how high the stakes are, Trump will return to Pennsylvania on Saturday to campaign for Oz, while President Joe Biden and former President Barack Obama will campaign for Fetterman that same day.
Oz left Oprah’s show after five years and 55 episodes to start his own daytime TV program, “The Dr. Oz Show,” which ran for 13 seasons before he moved from New Jersey to Pennsylvania to run for the Senate.
The seat is being vacated by retiring Republican Sen. Pat Toomey.