Secretary Kubus Oak was a Bajoran member of the Cardassian Occupational Government during the Occupation of Bajor. When the occupation ended in 2369, Kubus left with the escaping Cardassians and retired on Cardassia Prime.
In 2370, Kubus returned to Bajoran space to live out his last days on Bajor. After his transport docked at Deep Space 9, he was arrested by the station's Chief of Security Odo who recognized Kubus. His arrest was made legal by the Ilvian Proclamation in which his name was fourth on the list.
Kubus then presented Vedek Winn with circumstantial evidence that Vedek Bareil, not Prylar Bek, was the traitor responsible for the Kendra Valley Massacre. Winn granted Kubus sanctuary on Bajor so that she could use his testimony against Bareil. (DS9: "The Collaborator")