It was announced that DC Universe will be releasing a paid DLC that will allow those to gain themselves a power ring. DC has announced that though you will have the option to fight on the side of the Oa or the Sinestro Corps you will not officially be one of them. Though this shouldn't matter too much since it fits with the game's "almost" style. This being that your character is "almost" Superman, or "almost" Batman. Before you freak out not really being able to become a Green Lantern, consider the following. DC has said that the rings players receive are emergency rings that are duplicates of existing Green Lantern rings. Though they have not commented on it, this most likely is the excuse for the yellow power rings as well.
The DLC will also allow users to go to new locations such as Oa and Coast City, and has confirmed that you will be able to meet up with the other Green Lanterns of Earth (all confirmed except for Kyle Rayner sadly). There are also new quests that will be added to the game in the form of an attack by Brainiac, and an invasion by the Red Lanterns. DC has said that more Green Lantern content is on the way, and has confirmed that more of the colored warriors will be making appearances in the future. However, for now if you want to get your hands on a power ring you're going to have to shell out $9.99.