The pair met during ITV2's second series of the dating show last summer and have stuck together ever since.
In the new year, Alex flew Olivia out to New York before getting down on one knee.
"She said yes," he posted online alongside a snap of the pair back on New Year's Day.
Olivia added: "I have no words for how happy I am. My fiancé my everything @ab_bowen this was magical. So magical."
Now well into wedding planning, Olivia has revealed hopes of some famous guests at her nuptials.
"Caroline Flack needs to come to the wedding! She's a real party animal so we need her there," Olivia explained. "We're inviting all the Love Island producers too, as they made it happen - and of course Cara [De La Hoyde, who won the show] is going to be my bridesmaid."
Speaking to Closer magazine, Olivia went on: "Plans are going well! I'm the least organised person but I'm trying so hard to get everything going. The date is set for September 20187 and I've narrowed down the location options."
And she said of living with Alex: "I can't explain it - it's honestly amazing how we've found each other. We're so compatible - we're like the same person."
Meanwhile, Olivia ruled out starting a family with Alex for the time being, saying they had enough on their plate with a new puppy.
"We feel like we're parents to him and he can be a nightmare - so no kids just yet," Olivia shared.