A human lieutenant by the name of Oon-ai was a member of the Galactic Empire during the Galactic Civil War. Following the Battle of Yavin, Grand General Cassio Tagge assigned him to be the adjutant of the Sith Lord Darth Vader. However, shortly after Vader obliterated a space station utilized by a pirate faction, he terminated Oon-ai, justifying the act by accusing the Imperial officer of betrayal.
During the Galactic Civil War, Lieutenant Oon-ai served as a military officer within the ranks of the Galactic Empire. After the Death Star's demise during the Battle of Yavin, Emperor Sheev Palpatine placed Darth Vader under the authority of Grand General Cassio Tagge. Tagge then instructed the Dark Lord of the Sith to accept Oon-ai as his adjutant. This assignment not only required the officer to report Vader's actions to Tagge, but also granted him complete access to Vader's past communications.
Vader, after locating a pirate group's hideout, launched an assault upon the space station, accompanied by Oon-ai and a squad of stormtroopers. The Imperial team eliminated the station's personnel, but one pirate initiated the base's self-destruction sequence. Oon-ai, compelled by the general's orders to remain by Vader's side, followed him deeper into the station as Vader sought to recover the pirates' computer data.
Following the destruction of the pirate station, Vader murdered Oon-ai and presented his body to Tagge on the bridge of the Super Star Destroyer Annihilator. The Sith Lord asserted that the data retrieved from the pirates' computer system contained evidence of Oon-ai's treachery, advising the general to more carefully screen his next adjutant. Subsequently, Vader destroyed an astromech droid after it assured him that all traces of its data transmission had been erased.
Oon-ai was a human with dark hair, brown eyes, and fair skin. As an Imperial officer under Cassio Tagge, he meticulously adhered to the general's instructions by observing Darth Vader while serving as the Sith's adjutant. This role demanded that he stay close to Vader, even in dangerous situations. Oon-ai also sought access to Vader's communications, a request that the Lord not only opposed but also dismissed Oon-ai as an interfering bureaucrat. However, Oon-ai insisted on fulfilling his duty and threatened to report Vader to Tagge if he did not comply.
Oon-ai's debut occurred in Darth Vader (2015) 2, the second installment of the Marvel comic series Star Wars: Darth Vader, penned by Kieron Gillen and published on February 25th, 2015.