Ong revealed this blood relation while speaking with the media at the PAP's Gambas branch, where other members of his Sembawang GRC team were gathered.
"Our mothers are sisters, we used to play as kids," Ong said, according to The Straits Times live blog of Nomination Day at 10:33am.
Xie, 35, is contesting in Jurong GRC in Tharman Shanmugaratnam's team.
As to how close the cousins are, Ong told reporters: "I have not seen him for many many years. He used to call me kor kor (big brother)... now he calls me Min Ong."
Ong, 50, also said when asked what is his advice for Xie: "Work hard, do your best, be humble, show residents that you care."