They are the students and alumnae of the Oprah Winfrey Leadership Academy for Girls in South Africa, and to them, she is simply Mom O.

“They are my greatest, deepest joy,” says Winfrey.

Opened in 2007, OWLAG, as it’s called, is a boarding school with facilities rivaling those of top prep schools in the United States. More importantly, 90 percent of graduates have enrolled in colleges including Harvard and Oxford — remarkable in a country where roughly 20 percent of students attend university.

“I wanted to build a school for girls who are like me,” Winfrey tells PEOPLE.

Raised by her grandmother in Mississippi during the Jim Crow era, Winfrey understands how poverty can seep into someone’s bones.

She says her girls “have been in the heart and heat of poverty. Poverty itself is a traumatic event, just trying to be able to have food on your table every day. Most girls have lost one or another really close relative, sometimes both mother and father.”

Over the years, Winfrey and the young women have become more than mentor and students, they are a family. Through fireside chats and texting, Winfrey listens and offers advice to the girls as they wind their way through adolescence and adulthood.

When the girls began to call her Mom O, she was hesitant.

“When the girls started calling me Mom O, I did not want their mothers or their legal guardians to think that I was stepping in and trying to take over the role their mom mom holds in their lives,” she says. “I’m very much aware of who I am and what my role is for them.”

“They’re the daughters I did not have,” Winfrey, who’s turning 65 on Jan. 29, says. “I never thought that that mothering instinct was something for me. I like babies, but I like them like, ‘Hi baby, okay, now go over there.’ I like children when they can actually speak to me and tell what is the problem. This is how having children was supposed to manifest for me.”