http://www.techradar.com/news/face-unlock-arrives-in-beta-form-on-the-oneplus-5-just-in-time-for-2018

As promised, you can now unlock the OnePlus 5 with your face, as long as you're prepared to install the beta version of OxygenOS. The feature originally appeared as a OnePlus 5T exclusive, but will now work with the handset that preceded it as well.

Seeing as both phones have the same front-facing camera, the developers at OnePlus shouldn't have had too many problems getting the security tech to work on the OnePlus 5 too. According to company chief Carl Pei, the feature has been added due to "popular demand" from users.