The Book of Oa is a massive book held in the Citadel of Oa. Created long ago, the Book contains the history of the Guardians of the Universe and the Green Lantern Corps. One member of the Green Lantern Corps is selected by the Guardians as Keeper of the Book of Oa, a highly honored position.
After Thomas Kalmaku used Hal Jordan's ring to rebuild Oa, Kyle Rayner used his power as Ion to ressurect the Guardian. The book of Oa is now restored.
The Forbidden Chapter of the Book tells the prophecy of the Blackest Night, the final destruction of the Green Lantern Corps at the hands of their greatest enemies as it was told to Abin Sur by the demons of Ysmault.
Upon recruitment, a Green Lantern is expected to uphold certain principles of his/her/its duty. These principles include:
1. The protection of life and liberty within the assigned sector.(Revision allows lethal force to be used against the Sinestro Corps)
2. Following the orders of the Guardians without question.
3. Noninterference with a planet's culture, political structure, or its population's collective will.
4. Acting within local laws and obeying the local authority within reason. (Presumably, The Guardians' orders can overrule this when necessary).
5. Taking no action against anyone or anything until they are proven to be a threat against life and liberty.
6. Refusing to use the equipment, resources or authority of The Corps for personal gain.
7. Showing respect for and cooperating with other members of the Corps and the Guardians.
8. Showing respect for life which includes restraint of force unless there is no reasonable alternative.
9. Giving top priority to the greatest danger in the assigned sector.
10. Upholding the honor of the Corps.
Following the escalation of the war between the Green Lantern Corps and the Sinestro Corps, the Guardians have rewritten the Book of Oa changing the ten previous laws into new laws.
1. Lethal Force is authorized to be used against the Sinestro Corps.
2. Lethal Force is authorized against all enemies of the Green Lantern Corps.
3. Love and physical relationships between members of the Green Lantern Corps is forbidden. (Repealed)
4. The Vega System is no longer Outside the Green Lantern Corps' jurisdiction.
5. The Green Lantern Corps no longer takes prisoners.
6. If the Guardians are unable to discharge their sacred duties, command of the Green Lantern Corps falls to Clarrisi, followed by the Illustres.
The remaining four laws have yet to be revealed.
To enforce these principles, the Guardians closely monitor the activities of the Lanterns. If they feel a violation of Corps regulations occurred, they will summon the offender to Oa and hold a trial in which the charges are read and the Lantern is allowed to explain his/her/its actions. If the Guardians are not satisfied by the explanation, they have a number of disciplinary options which include:
Personal supervision by the Guardians on Oa
Temporary exile from the Lantern's homeworld
Ritual Trial of Endurance - a Lantern must attempt a dangerous passage through the Anti-Matter Universe.
Expulsion from the Corps.