An opinion leader is someone whose views and opinions are followed closely by others who will often tend to come to agree with those views. Oprah Winfrey became an opinion leader by virtue of her charisma and her ability to attract and hold viewers/listeners.
Winfrey did not become an opinion leader by holding an elected position or other such official post. Instead, she became an opinion leader by attracting a large audience to her radio shows and then to her TV shows. She was only able to accomplish this because of her innate talents and her willingness to work hard.
Once she had gained this audience, Winfrey could be an opinion leader. She had the ear of millions of people and had gained their trust through all of her shows that they had seen. This gave her credibility with them and gave her the chance to influence their opinions.