Ocato of Firsthold (also known as High Chancellor Ocato) is an Altmer who served as the Imperial Battlemage of Emperor Uriel Septim VII and the High Chancellor of the Elder Council and after the Oblivion Crisis as Potentate of the Empire.
After Emperor Uriel's assassination in 3E 433, Ocato became the de-facto leader of the Empire as the Head of the Elder Council. As Gates to Oblivion started opening around Tamriel and Daedra following Mehrunes Dagon began to invade, it was largely Ocato's job to hold the Empire together. Some of Cyrodiil's nobility, including Count Regulus Terentius of Bravil believed that Ocato intended to use the Oblivion Crisis to crown himself Emperor and that he did not truly care about Cyrodiil.
When the Hero of Kvatch, sent by Jauffre, the Grandmaster of the Blades, approached Ocato to ask for aid for Bruma to protect Emperor Uriel's last remaining heir, Martin Septim, Ocato refused. Ocato claimed he wished to help, but that he would have a political crisis on his hands if he would try to move any troops from other provinces to protect his interests in Cyrodiil.
After Mankar Camoran's death and the retrieval of the Amulet of Kings, Martin Septim came to the Imperial City to claim his title of Emperor. Ocato had already considered the matter with the Elder Council, and happily accepted Martin as the Emperor. But before they could complete the ceremony to relight the Dragonfires and reestablish the barriers between Nirn and Oblivion, the Imperial City was attacked by Daedra, led by Mehrunes Dagon himself.
Ocato helped Martin fight his way to the Temple of the One, but they were too late; the barriers between the realms were gone. Martin, however, used the divine blood within the Amulet of Kings to transform himself into an Avatar of Akatosh and banish Dagon back to Oblivion. Unfortunately, this caused Martin afterwards to turn into stone, destroying the Septim bloodline.
After the battle, Ocato names the hero who helped Martin the Champion of Cyrodiil and provides them with a full suit of Imperial Dragon Armor. Ocato continued to serve as the de-facto leader of the Empire, now having to deal with several provinces that threatened to break their ties to the Empire.
The books Rising Threat, Vol. III and Rising Threat, Vol. IV reveal that while Ocato searched for an Emperor, he reluctantly agreed to become the Potentate of the Empire until Imperial rule could be reestablished. After almost a decade, Ocato was contacted by Lathenil of Sunhold about the Thalmor; taking a greater interest in what he had to say than most. However he was later assassinated by the Thalmor, his death causing the Stormcrown Interregnum which fractured the Elder Council.