No. We do not have any exchange offer for Oppo F9 Pro as of now. In fact, if you sell them via Olx or Quikr - chances are there, that you will get better value for your old phone.
No. Oppo F9 Pro does not have the NFC support.
Yes, Oppo F9 Pro does have the OTG support.
Oppo F9 Pro does not have the front camera LED flash but it does have the display screen flash which helps to take good selfies in low light conditions.
The Oppo F9 is among the most attractive in the mid-range today. Unique with that waterdrop-like notch and beautiful with the gradient back with cool pattern underneath.
The Oppo F9 Pro reminds us of the early 2000s when Nokia, Samsung, Sony Ericsson, and others used to come up with wildly imaginative designs all the time. Very few phones these days take any risks with colours, patterns, shapes or materials.