One of the new features on the OnePlus 5T is face unlock, which works in a similar manner to the iPhone X's face unlock (though OP's implementation is less secure). OnePlus 5 owners already received an early Christmas present in the form of an Android Oreo update, but there is another addition in the works - face unlock support.
Carl Pei, Co-founder of OnePlus, tweeted the surprise announcement earlier today:
Due to popular demand, Face Unlock is coming for the OnePlus 5. Thanks for the feedback, and merry Christmas! 🎅🎄🎁
— Carl Pei (@getpeid) December 24, 2017
Since the OnePlus 5 and 5T are very similar from a hardware perspective, it probably isn't surprising that the feature would work on the previous model. As far as I can tell, face unlock is not in the Oreo update (it's not mentioned in the changelog), but let us know in the comments if you have updated and see the option.