The 38-year-old X Factor host proposed to his TV presenter and producer partner during a mini-break in New York last November.
O'Leary confirmed their engagement on The Xtra Factor after the spin-off show's presenter Caroline Flack announced on air: "We want to congratulate you... on your engagement!"
Speaking ahead of his wedding during a recent interview, O'Leary had said: "Wedding plans are going well and engaged life is brilliant. We lived together for seven years and been going out for 10 years so we are settled down already.
"There's no sort of raucous bachelor days to put behind me when we get married. I love my girlfriend and the woman that will be my wife and we are really relaxed with each other."
Koppang wore a vintage-style lace wedding dress with fishtail kick at the intimate ceremony in Chiddington.
Guests were said to include Lauren Laverne, Caroline Flack and One Direction.