Yes, Oppo F9 does have the Corning Gorilla Glass protection.
Yes, Oppo F9 is a dual VoLTE mobile phone. You can use 2 4G network sim cards at the same time.
Yes, Oppo F9 does have the OTG support.
Yes, Oppo F9 does have the dual rear camera set up. The main camera has 16MP resolution while the second rear camera has a 2MP resolution.
The most striking feature of the Oppo F9 is the big display with the waterdrop notch. It is well designed, delivers good battery life, and offers decent performance for the price.