The Oromo are one of the Cushitic speaking peoples and the largest single ethnic group in Ethiopia inhabiting most areas of Ethiopia. The Oromo people had enjoyed almost a homogeneous culture and shared a common language, history and descent. They once had a common political, religious and legal institution. Though the exact time of the evolution of the Gadaa institution among the Oromo is not known, it was long before 16th century that the Oromo practiced such egalitarian socio-political and religious system. The system guided the socio-religious, political and economic life of the Oromo. It was the constitution of the society through which the Oromo administered, defended their territory, maintained and developed their economy. The Oromo used to practice Gadaa rituals, religious ceremonies and conducted political activities under the shade of Odaa tree.