No. OPPO A37f is not waterresistant.
Yes, OPPO A37f does have OTG support.
Yes, If your budget is around Rs.10,000 then Oppo A37 is one of the best phones, which you can buy. Oppo is very well known for its camera phones and better quality than many other brands. It has got expandable memory card slot and you can expand the memory up to another 64GB via MicroSD card. It is VoLTE enabled and you can use Jio sim card in it.
You can use your credit card to buy OPPO A37 on EMI.
Once again Oppo has presented us with a device that is functional, to the point and performs just as expected.