As Chamberlain and Royal Genealogist, Otho's duties required him to study and maintain the bloodlines of the royal house to ensure they would continue to be able to use the Tower's Throne Chair to defend the planet from the Wraith and maintain power. Otho also gained a basic knowledge of some of the Ancient devices left behind in the Tower including a Life signs detector he could use and abandoned Puddle Jumpers. Otho spent a lot of time making various plans, supposedly for the benefit of the Royal House but more for himself and studied poisons to the point that he gained "a fair knowledge" of them.
Otho eventually fell in love with the Lord Protector's daughter Mara and sought to marry her. To this end, Otho made a deal with Tavius where he would help Tavius assassinate his father and take the throne in exchange for Tavius sparing Mara's life and allowing Otho to marry her. Otho then poisoned the Lord Protector over the next few months through his food until the Lord Protector was fatally ill and no one was the wiser to Otho and Tavius' plot. (SGA: "The Tower")
When the First Atlantis Reconnaissance Team arrived, Otho was sent to greet them and discovered through his scanner that Lt. Colonel John Sheppard had the strongest Ancient Technology Activation gene seen in over 300 years. After the Lord Protector coerced the team into giving up their weapons with a harmless drone weapon attack, Otho took Sheppard to the Tower to meet the Lord Protector.
While Sheppard was in the Tower, Otho put on a mask of friendliness, explaining the situation with the bloodlines to Sheppard after Mara propositioned him and showing Sheppard the inert Puddle Jumpers. When Sheppard offered to have Dr. Carson Beckett look over the Lord Protector, Otho agreed so as not to draw suspicion upon himself, something he explained to Tavius when confronted by him. Sheppard also offered the solution of the Ancient Technology Activation gene therapy which drew Otho's interest as it would be a means for seizing power for himself once the Lord Protector was dead.
Although Beckett eventually realized the Lord Protector had been fatally poisoned and Sheppard realized that Tavius had a hand in it, neither suspected Otho who told them to flee after the Lord Protector died. However, Beckett left the gene therapy with Otho who took it and gained the ATA gene. No longer needing Tavius, Otho had him arrested for the murder of the Lord Protector and arrested Sheppard and Beckett upon learning of the uprising started by Ronon Dex and Teyla Emmagan in the nearby village. Otho finally revealed his true colors to Sheppard and Beckett and admitted his part in the murder plot against the Lord Protector, telling them he intended to destroy the village to prevent anyone from rising against him again.
Following the revelation of the truth, Otho takes up the mantle of Lord Protector and launches a drone attack on the village from the Control chair, confident in his own power. However, Dr. Rodney McKay is able to deplete the Tower's Zero Point Module by charging the city's Stardrive, rendering the drones inert and foiling Otho's attack on the village.
After the failure of his attack, Otho is enraged when Sheppard and Beckett arrive and realizes what they did. Otho orders his men to seize Sheppard and Beckett, but Sheppard reminds Otho that he now has no power without his ability to control the drones. As Sheppard tells everyone that they no longer have to bow to Otho or anyone else, Otho draws a knife to kill Sheppard with. Thanks to a warning from Mara, Sheppard is able to dodge the attack and yank the knife from Otho's hand, cutting him across the palm in the process. Otho tells Sheppard "Tavius was right about one thing: I do have a fair knowledge of poisons" before collapsing dead from the fast-acting poison he'd laced his knife with which had gotten into his system when Sheppard cut him. (SGA: "The Tower")
On the outside, Otho appeared to be a kind-hearted man with the best interests of all of his people in mind, even stating that he did not intervene in a whipping attempted by the Lord Protector's Guard to maintain his image. He appeared concerned about the Lord Protector's health and was willing to agree to Dr. Carson Beckett trying to help him and was concerned about what would become of his people once the Lord Protector died. He was also shown to be highly intelligent and intuitive, appearing to have a better understanding of the Tower's systems and purpose as shown with his ability to use an Ancient scanner, his recognition of the true nature of the Puddle Jumpers and his comments to Sheppard on what he suspects about the Tower.
However, in truth Otho was a cunning and power-hungry man who was more than willing to go as far as mass-murder to secure his power once he got it and cared for no one but himself and Mara due to his infatuation with her. According to Tavius, Otho was always making plans and strategies that he claimed were for the good of the Royal House, but were in reality for his own betterment. Through study, Otho gained "a fair knowledge" of poisons, something he was greatly proud of and used to great advantage, being able to poison the Lord Protector undetected over the course of a few months and coating his personal knife with a lethal fast-acting poison so any enemies even just cut with the knife would die. (SGA: "The Tower")