OnePlus has done its best to keep its devices as close to stock Android as possible. One of the newest features the company is introducing is a deeper integration of Google Duo into the Phone app. Google has this integration in the Pixel dialer, but since OnePlus uses its own app, the company decided to add its own take on the feature. As of OxygenOS 9.0.12 on the OnePlus 6T and OxygenOS 9.0.4 on the OnePlus 6, 5, and 5T, there will be a video button visible alongside names in your contacts as well as in the call log. OnePlus 3 and 3T will be getting the feature once the Android Pie update reaches these devices.
The inclusion came about because of the users' request. The company did a survey with OnePlus users in India and found that Google Duo ranked highest in terms of video call quality. The traditional carrier video calling feature will remain available through the option “Contact > Select contact > See all > Choose video call."
Source: Phandroid