Face unlock isn’t a new feature, but ever since Apple announced Face ID with the iPhone X, the feature has gained mass appeal. The OnePlus 5T has the fingerprint sensor relocated at the back of the smartphone and the front camera works with Face Unlock.
Towards the end of 2017, OnePlus announced that the feature would be available on the OnePlus 5 and it is now available with the Oreo based OxygenOS open Beta 3. OxygenOS Beta 3 brings with it a new security patch and some general optimisations.
The Face Unlock on the OnePlus 5 works just like it does on the 5T. The feature allows you to unlock your phone almost instantly by just looking at it. It's not as secure as the fingerprint sensor, but it is extremely convenient.
If you want access to the open beta now, you will have to flash it on your OnePlus 5. If you have enrolled for the beta, you should get the update via OTA.
This Face Unlock feature shouldn’t be confused with trusted face. The Trusted Face was introduced by Google to Android quite some time back. However, while some OEMs have disabled the feature, it is available on devices running stock Android such as the Moto G5s Plus and the Xiaomi Mi A1. You can read more about Trusted Face here.