Face Unlock is a feature which was added at the beginning to OnePlus 5T, and people liked it very much. Face Unlock option works in the similar way like front camera in iPhone X, but in OnePlus 5T this function allows only to unlock your phone while in Apple's device you may use this function in other aspects. Despite this, the new function of the OnePlus 5T is used by many users, which prompted the creator of this phone to implement this option to the predecessor of 5T - OnePlus 5.
This information was confirmed by Carl Pei, co-founder of OnePlus on his Twitter account. We don't know when exactly Face Unlock will be added to OnePlus 5. In the latest update of OxygenOS we haven't found this feature yet.