'Mom Oprah' Bound to School's Grads They lovingly call her "mom Oprah." Oprah Winfrey may not have biological children, but thanks to the first graduates of the Oprah Winfrey Leadership Academy for Girls, she said she now understands the trials and tribulations, as well as the joys, of motherhood. "We're texting all the time," Winfrey told "Nightline" in an exclusive interview. I was on the phone last night for an hour with one of them. Watch the exclusive interview with Oprah Winfrey on "Nightline" tonight at 11:35 p.m. The maternal bonding began five years ago, when she handpicked 72 underprivileged girls out of thousands of applicants from shantytowns across South Africa to leave their homes for an opportunity at a top-notch education -- an idea inspired by Nelson Mandela. What is the single, life-changing, trajectory-changing moment for me was being exposed to a world of education that offered me insight into how I saw the world, and I just wanted to do that for somebody else. That's all," Winfrey said. Five years later, the girls are in colleges in the United States. Like any proud mom sending her kids off to college, Winfrey said she got emotional over the idea of furnishing their dorm rooms. "The very first morning that the girls from the United States were here and we were going-- I can't believe I'm weeping over Target and Bed, Bath and Beyond," Winfrey said. But the morning that we were all getting dressed to go shopping at BBB for their college dorms, I mean, it just [was] the moment it hit me that all the pain, really, and challenges, and difficult circumstances, and the crisis we went through in 2007, and all the questions and times I thought about, 'Give them the money and step out of this and let them have it,' it all came to a head, a joyous fruition. According to Forbes magazine, Winfrey has devoted $400 million to the girls' education in the U.S. and abroad. Her emotional journey involved in building the school is chronicled in a documentary called, "The First Graduating Class: Oprah Winfrey Leadership Academy for Girls," a two-hour special that will air on the on OWN, Sunday at 11 p.m. Winfrey said she wanted to make a point of donating not just money to fund the Leadership Academy project, but also her time and energy. In the process of going back and forth to South Africa, Winfrey said she developed meaningful relationships with the girls, who had experienced the struggles of living in poverty -- not unlike Winfrey, herself, who grew up poor and attended segregated schools in Mississippi. To this day, Winfrey said, she will stop whatever she is doing to take a call or answer a text message from them. "I'm in a really, really, really good space," Winfrey said. I will tell you what has brought me the greatest happiness -- so really, really unexpected. Being able to step into the mother role and be a real mother friend companion advisor, comforter to these girls has added another level to my life that really just came as a surprise to me. I didn't really know that I would love it so much. One of the most poignant moments for Winfrey when the school was being built, she said, was watching the girls realize they had plumbing for the first time. "When you are in your home, and you have nice tile, and you have carpet on the floor, and you have an aesthetic of beauty and an appreciation that says, 'You are valued and you are valuable,' and that was what I was most excited about -- 'Notice the artwork, girls,'" Winfrey said. But they were most excited about the plumbing, because it means, 'I can take shower, I don't have to find buckets of water that are two kilometers away, I don't have to share a pump with 56 other people, I can have my own bed, I can flush the toilet.' The journey didn't come without its hardships. Shortly after the school opened in Meyerton, South Africa, a town outside of Johannesburg, in January 2007, Virigina Tiny Makopo, a school matron, was charged with sexually molesting several girls. At the time, Winfrey flew to South Africa to personally apologize to the students and their families. She also fired the school's headmistress, Lerato Nomvuyo Mzamane. Makopo was acquitted of all charges in October 2010. In the wake of the scandal, Winfrey decided to re-double her commitment to her young students instead of quitting. "It takes a level of commitment and leadership, and a commitment to sustainability in the long term that makes all the difference," she said. None of the girls who attended Winfrey's school had financial support to live away from home, although both they and their families understood the life-altering gift of education. Now, as they prepare to continue their education at the college level, Winfrey is still mothering, still providing support for them. "I knew from the moment that I met these girls that I was going to fall in love with them, and I did," she said. When you change a girl's life, you change not just her -- you change the whole family.