Pop Goes The News — Dr. Mehmet Oz kept viewers in the dark when he used his TV show to promote a company partly owned by his boss, Oprah Winfrey.

The first guest on the episode that aired Tuesday was Winfrey’s best friend, broadcaster Gayle King, who was on to talk about her weight loss plan.

“This is my third time at Weight Watchers,” King told Dr. Oz, later gushing about the effectiveness of the program.

Neither Dr. Oz nor King disclosed to viewers that Winfrey — whose company Harpo Studios co-produces The Dr. Oz Show — is the third largest shareholder in Weight Watchers.

Winfrey paid $43.2 million for a 10 per cent stake in Weight Watchers in 2015 with options to acquire an additional 5% at a fixed price. She also sits on the company’s board.

At the end of 2016, Winfrey owned 6.4 million shares, worth a total of about $77 million.

The 63-year-old billionaire appears in commercials for Weight Watchers, which she credits with helping her lose more than 20kg. The commercials do not disclose that she is an investor in the company.

In a tweet on Oct. 19, 2015, Winfrey announced that she invested in the company and joined its board. But she has since regularly tweeted plugs for Weight Watchers to her 36 million followers without noting her stake in the company.