The star, 25, broke the happy news on her Instagram account, sharing a series of pictures of the couple posing together in front of the backdrop of St Paul's Cathedral.
It seems Oliver, a model for high-profile brands including Marks and Spencer, caught Pixie totally off guard with the romantic proposal, as she captioned a second snap of her impressive diamond band: 'So happy ❤️️ biggest surprise ever'.
Oliver shared his own picture of the pair, calling himself a 'lucky man'.
'What a beautiful day and the most beautiful woman in the world said yes to me on it,' he wrote.
Oliver and Pixie have been dating for six years after meeting at a fashion show.
In 2014, she told The Mirror that she wanted to get married to Oliver, but felt the pair were too young at the time.
'I do really want to get married – and have wanted a double wedding with my sister but she went ahead and got married without me!' she explained.
'As much as I like Oliver, we're not ready to tie the knot just yet. We've still got so much we want to do.'
Speaking to PennyBlackMusic.co.uk the following year, she said of her now fiancé: 'Oliver is my first serious relationship.'
'Of course, I had some flings before, but I was never in love. Maybe I thought I was sometimes, but I'm sure now that I wasn't. Oliver is definitely the real deal. I am very happy and it feels just right. Though it's not always easy as we both travel a lot. Thankfully we both Skype!'
'I always wanted to get married. Even at nursery school I used to run in and go straight to the dressing up box and put on a wedding dress.'
'I come from such a strong family so, yes, I definitely want marriage and babies. That is what we are all on this earth for, isn’t it?'