Oh served the military as an officer soon after graduating from law school. After finishing his military service, he worked as a lawyer in Seoul for about 15 years. He appeared on television programs about law many times. That made him become very famous, so he campaigned to being a politician. In 2000, he was elected as a member of the National Assembly, belonging to Grand National Party. As a young politician, he tried to break and change the old and conservative image of Grand National Party. After completing the four years of his term as a member of the National Assembly, he went back to being a lawyer.