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 the couple are well into wedding planning for 2018 and are hoping for a big Love Island themed celebration.
"It will be glamorous and beautiful with loads of flowers. My mum got married last year, so she’s going to be really involved with the planning," said Olivia.
The blonde beauty revealed host Caroline Flack would be at the ceremony, declaring: "She's our party animal friend so she has to come. I feel like the whole of ITV needs to come!"
The couple will also be inviting some of their Love Island co-stars, which could prove awkward for recently split Nathan Massey and Cara de la Hoyde, who won last year's show.
"It's like a year away so I'm hoping they'll stay amicable," Olivia explained. "They're both our friends so we'd love to have them both there. I can't see that there would be any bad blood.
"It is sad [they've split] but at he same time I'm like, 'If it's right for them...'. We still see them and nothing will change."
Meanwhile, discussing the upcoming third series of Love Island, Olivia told OK! Magazine: "It's bittersweet. Obviously we love it - we met each other an I've got a fiancé, so the memoires will still be there.
"It might be hard to watch it at first but I can't wait to see it."
And as for the future with Alex, Olivia said there were no plans for children just yet.
"We're enjoying everything we're doing at the moment. We want to be stable and happily married in five years time," she said.