World Line O (世界線O) is a different world line from the one the main characters live in.
According to the Game Master, World Line O split off around 100,000 years ago. Unlike World Line N, people that live in this world have horns on their head ranging from one to two, giving them an appearance which Kaname likens to Oni. Japan in this world seems to be in the Meiji era which Tanya refers to by stating that they have 1800s level technology. Buildings have traditional Japanese architecture as well as more modern looking buildings, though they don't have skyscrapers or massive amounts of people. According to Kouu, World Line O hasn't fought a major war in 100 years, leading to their technology being somewhat lacking, and implies that they had fought against World Line N 200 years ago.
The army of World Line O is split into three groups: the imperial guard, which is under the control of the royal family, the army, which is under the control of the regional shizoku, and the navy, which is under the control of the merchants. Members of the imperial guard wear uniforms which Long Tao calls retro by World Line N standards. Of the three, only the officers of the imperial guard have been shown to use sigils, which they refer to as koujinryoku (荒神力). There seems to be people that don't like the imperial family monopolizing the koujinryoku as they believe sharing it to everyone would bring equality, though it is unknown if they actually think that as one of Vladimir's supporters backed out when they realized who they were dealing with.
Outside of Japan, the United States doesn't exist, as the Revolutionary War hadn't happened, and there has been a revolution in eastern Europe, resulting in the Great Russian Republic which is said to be similar to the Soviet Union. The Republic has sent three agents to Japan in order to increase the numbers of sigil users, and they refer to sigils as Exen (エクセン).