Yes, Oppo F9 does have the double channel LPDDR4X type of RAM.
No. Oppo F9 is not a water-resistant mobile phone. Be careful !!
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.