Oromo society was structured in accordance with this Gada system. The Oromo society was structured into two distinct but cross-cutting system of pear group. One is the system in which the members of each class are recruited strictly on the basis of chronological age which the anthropologists call Age Sets. The second is a system in which the members are recruited on the bases of genealogical generation. This second one has little to do with age where as the first has nothing to do with genealogical ties. Both social groups pass from one stage of development to the next every eight years. The chronological age group is called Hiriya (age sets) while the genealogical generation group is called Luba (Gada class). The members of the class maintain their identity as a pear group throughout the life cycle.
At each level of the development or Gada grade the classes hold corporate responsibilities. The transition rituals by which the genealogical social group (the sons) passes from one grade to the next is performed every eight years during the life cycle. Every transition ritual at different levels has its own unique character, name and procedure. For example transition from the first grade (called Dabballe) to the next grade at eighth year of the life cycle (Gamme grade) consisted of two events- the hair shaving and the giving of names for the whole members of the grade. The principal responsibilities of the initiates or the boys after their rite of passage is the responsibilities of performing minor jobs as looking after livestock and horses. And they begin to grow a new hairstyle to represent their new status.
The transition from the third to the fourth grade is one of the most important events of the Gada institution. This transition rite is an isolation ritual which followed by election and proclamation of young Gada leaders which is purely political. The future political leaders of the Oromo are elected at this stage. All the boys in grade IV formally become members of Gada class at this stage and, with the induction of their leaders, the group becomes institutionalized as a corporate body. There is deep personal identification between the members of the class and their leadership. Thus the young men develop a new social identity that cut-across kinship ties. The election of Gadaa councilors at this stage also serves as transition from grade three to grade four.
Transition from grade IV to grade V at 32 year of the life cycle is marked by marriage of the whole class. Those elected young Gada councilors and other members of the class in their twenties and early thirties marry at this stage. The fifth grade is collectively called the warrior class. The main duty of this class is military service. Before they enter the Gada period or before they come to power they have to prove their capacity in military activities. At forty years of the gada cycle i.e. at the end of the first half of the Gada cycle there is a fatherhood ritual. This is the turning point in the Gada cycle. It is a border between generations. That is forty years long. Hence the fatherhood ritual is also called Dannisa marks the transition from one generation to the next.
The sixth grade is called the Gada grade. Here the term Gada is the stage and a period of eight years during which the elected government stay in power, not the whole Gada system. It is the most important stage of the Gada classes/grades. Passage into sixth grade or Gada (stage VI) is known as Baallii walirraa fuudhuu – walitti dabarsuu (handover –takeover ceremony of political power.) There is a stiff taboo against retaining political power/authority longer than the prescribed eight years. The day, month and year of handover/takeover ceremony is prescribed by ayyantu, experts in Oromo history and calendar. The outgoing and incoming Abbaa Gadas (heads of the Gada political leaders) exchange milk and blessing. The symbol of authority is Bokkuu (sceptre) and ostrich feathers. They also exchange these symbols. It is the stage of the incoming of Gada leaders who were elected during transition to grade IV, 16 years ago. On the whole this stage is the stage of political and military leadership. The investiture of Gada leaders at this stage is distinguished from the induction of Gada councilors during proclamation ceremony in the 24th years of Gada cycle .The Lalaba (proclamation ceremony) was purely an event of a group or a class. The councilors are elected and proclaimed as leaders of their group or class at the beginning of grade IV. When they reach Gada (Grade VI) after sixteen years they become leaders of all the classes and the whole nation.
In the Gada political system leaders who are unable to accomplish their duties are uprooted. The term in Oromo is called “Buqqisa” means uprooting. The distribution of power among the various offices in Gada council is based on equality. They are also fair in recruitment. There is little internal differentiation of functions except the separation of spiritual and political domains. There is little to do with “who does, what and when” type of questions in the system of bureaucratic organization. There are minimal ranking between senior and junior councilors. The existing differentiation of functions and minimal ranking does not imply inequality or hierarchy. Should a member of a council is absent when an important activity is in progress or important issue is discussed the activity or the meeting must be postponed until the missing councilor is summoned. However there is penalty for his absence.
The Gada Assembly (Yaa’ii Gadaa) is the highest authority or political organ in the Oromo’s Gada political system. What the Gada Assembly decides cannot be revised by any other authority. Gada Assembly is an ultimate authority. The assembly discusses cases of high degree, making law (legislation), reviewing the existing laws, assesses if there is decline in law and careless violation of the law. The major activity of the Gada Assembly can be described as legislative. It takes place during the fourth year of the eight years of the Gada period following the investiture of Gada leaders. Participants of Gada Assembly are Gada leaders, junior councilors, assistant officials in the administration, Yuba (semi-retired Gada leaders), many age sets councilors and a large number of clan elders. The Abbaa Gadaa (head of the government) in power presides over most of the general meetings. However, whenever conflict which concerns the presiding Abba Gada is discussed he is replaced by any other Gada councilor or semi retired Gada leader.
There are also abundant ethnographic and historic evidences indicating that the Oromo had an effective military organization, not only in the sixteenth century but also in the subsequent centuries until the conquest of Emperor Menilik of Abyssinia in the late 19th century. In fact by African standard the Oromo had effective military organization headed by military officer titled as Abba Dula (War chief). The Oromo had all that in the 16th century, and continuing, in varying degrees until they fell under Abyssinian rule in the late 19th century. The Oromo had a powerful cavalry that stood at the head of their military campaign. Oromo horsemanship is a highly prized aspect of their culture and it had a great deal to do with their military success over the centuries. Comparable in its magnitude to the powerful Zulu nation of South Africa and the Fulani of West Africa the Oromo were superior in military establishment. The Zulu and the Fulani though they had military strength they had no horses and cavalry forces.
The most remarkable account of Oromo military organization is to be found in the Chronicle of Emperor Sussenyos (1600-1625) who in his youth grew up among the Oromo as war captive. Later he used Oromo warriors and military strategy to take the imperial throne. The Chronicle describes that the Oromo army was structured into regiments called Cibra (Chibraa). These were age regiments consisting of the married Gada class members, Qeerroo (bachelors) and qondala (youth). The three age regiments in other words are age-sets belonging to grade IV, V and grade VI of the Gada stages of development. There were many age sets under each grade and many Cibras under one age set. Heads of each Cibra are called Abba Cibra. There are also different units in most cases nine units under one Cibra. It was the introduction of fire arms into the region by Europeans that changed the balance. The Europeans provided the Abysinian chieftains with then modern fire arms and military advisors that put the Oromo at a great dis-advantage.
Following handover of political power the outgoing Gada leaders enter the Yuba grade (semi retired). There are four stages of Yuba that is grade VII, Grade VIII, grade IX and grade X are the stages of partial retirement with varying degrees. The Yuba or semi-retired covers 32 of the life cycle (48-80 years of the life cycle). Their positions are linked with those of their sons. Their position is determined by the position of their sons. One of the roles of the Yuba (semi-retired) classes are that they are main protagonists in the election campaigns. They campaign on behalf of their sons. This can take place at the juncture of grade VIII-IX on the one hand and at the juncture of grade III-IV on the other. The Yuba take part in Gada Assembly and indeed have some residual political power in the Assembly. They may be required to serve as assistances unless Gada leaders announce him completely retired a man remain liable for political and social service of the nation.
After eighty years of the Gada cycle the Yuba enter the stage of Gadamojji (complete retirement) Gadamoji is the eleventh and final stage of the gada grades. The Gadamoji are similar to the Christian “ monks” because their life resembles the monastic life of the monks. The transition into this final grade is formally take place by rite of incense exchange (Qumbi wal irraa fuudhuu)
One generation in Oromo covers 40 years of the life cycle. The first forty years of the total life cycle of eighty years is an active generation which is divided into five segments each having a time spam of eight years. The basic rule of the Gada system is that the newly born infant child always enters the system of grades exactly forty years after his father, regardless of the age of the son or the father. Father and son are five grades apart at all times. Consequently, the Gada class incorporates within its ranks people varying in ages. The infant joins, sometimes, very old men who are members of the genealogical generation. Extremely young children can be witnessed dressed in adult ceremonial costumes, taking part in rituals and being treated as equals by adults. There are several inter-generational rules of the Oromo society under the Gada system of which the basic rule is the position of a man on the Gada cycle is always five grades (forty years) ahead of the grade of his son.
As late as the 19th century, the Gada system was still a living institution among the Tulama Oromo in central Oromia. Today the Gada institutions have been preserved among the Borana and Guji clans of the Oromo nation. There is some historical evidence that in the 16th century , one system of Gada class governed the whole of the Oromo nation. Between the 17th and 19th centuries however, the major divisions of the society drifted apart, each evolving slightly different versions of the same institution in different branches of the society. The centrality of the Gada system in Oromo social life gradually declined in many parts of Oromia and Gada political activities progressively transformed into simple life crisis and rituals. The Oromo socio-political structures had been dismantled by the Abyssinian conquerors that came from the north. The Oromo political institutions and social fabrics have suffered a lot from the Abyssinian cultural and political domination for the last hundred and fifty years.