One of the main selling points of OnePlus 5T is the face unlock function of Face ID, except for the screen ratio of 18:9. According to Carl Pei’s post on Twitter, this new function will be used in OnePlus 5.
Carl did not say when the function would be pushed to OnePlus 5, or whether it would be any different from the 5T. But because OnePlus doesn’t have extra sensors on the phone, it has to rely on the front-facing camera for face recognition. Security is not its main feature, just easy to unlock.