Her talk show regularly draws seven million viewers, nearly twice as many as the next biggest talk show, “Dr. Phil.” Her endorsement of Barack Obama is widely credited with helping elect him president in 2008. She even claims to own the trademark on the phrase “Aha moment.”

Ms. Winfrey, 55, told the staff of her Chicago-based show about her decision late on Thursday afternoon, according to people who were told but who insisted on anonymity because they agreed to wait until Ms. Winfrey makes the official announcement. Then she informed her business partners at the networks and instructed Harpo employees to start calling her affiliate stations. The stations were told they could report her decision on local newscasts, apparently to increase audiences for her formal announcement on Friday.

It remains unclear what on-camera role Ms. Winfrey will have at OWN, which is a 50-50 joint venture with Discovery. The management team at OWN has been busy creating a programming plan but has remained mostly silent about the lineup in deference to Ms. Winfrey and her decision-making process. Discovery executives declined to comment on Thursday evening. The OWN channel will have its premiere in January 2011, according to a person with knowledge of Ms. Winfrey’s decision who insisted on anonymity.

Ms. Winfrey made clear to her staff members that she will not transfer the show to cable. She is expected to produce new programs for OWN, and will appear occasionally on some of them.

Ms. Winfrey was believed to be in renewal talks this fall with CBS Television Distribution and another syndicator, Sony, which distributes her most recent spinoff, “Dr. Oz.” At the same time, she considered ending her talk show altogether. In recent days television executives said they sensed that Ms. Winfrey was leaning toward an exit.

CBS seemed eager to keep its door propped open for Ms. Winfrey, saying in a statement that, “We look forward to working with her for the next several years, and hopefully afterwards as well.” Acknowledging OWN, they added, “We know that anything she turns her hand to will be a great success.”

Image Whitney Houston, left, being interviewed by Oprah Winfrey. Ms. Winfrey will focus her energy on her cable channel. Credit... George Burns/Harpo Productions, via Associated Press

OWN is expected to replace the Discovery Health Channel, which is currently available in more than 70 million homes. Her cable plans, however, have been fraught with delays. OWN was announced in January 2008 and was originally expected to make its debut this winter, but those plans were scuttled because of management turnover and a turbulent advertising climate.