There is much difference between the price of these phones. Specifications-wise the OnePlus 6T is better but the Oppo R15 Pro is also great at its price.
Processor: The OnePlus 6T comes with the Qualcomm Snapdragon 845 processor which is better than the Qualcomm Snapdragon 660 MSM8956 processor of the Oppo R15 Pro.
Camera: The OnePlus 6T has a camera of 16 MP + 20 MP with OIS feature whereas the Oppo R15 Pro has a camera of 16 MP + 20 MP without OIS.
Check Full Comparison Oppo R15 Pro vs OnePlus 6t
Oppo R15 Pro comes with a Dual Camera package consisting of a 20 MP+ 16 MP.
The device also comes with 20 MP of front Selfie Camera.
The picture quality of the device is quite nice and detailed.
The device performs quite well during low light images also( Keeping the price bracket in mind ).
In the meantime,do not forget to find all the detailed specifications and other latest updates regarding Oppo R15 Pro here.
Oppo R15 Pro has got a 3430 mAh battery with Vooc charging support. It can charge from 0 to 100% in just 35 minutes.
You can see detailed specs here: Oppo R15 Pro