OPPO A37f is a VoLTE true 4G mobile phone.
Yes, OPPO A37f does have OTG support.
Yes, you can change themes on OPPO A37 very easily. OPPO A37 comes with OPPO theme store. There are many options available - both free & paid ones. Choose the one which you want to use. If it is free, you will be able to download immediately else you have to pay for it. Select the desired theme and click on Apply button.
Yes, OPPO A37f is a true 4g volte enabled dual sim mobile phone.
Once again Oppo has presented us with a device that is functional, to the point and performs just as expected.