Can you also zbs. Buy a Huawei, Apple headphone adapter and then fits the OnePlus phone? Or do you have to know exactly the brand?
There are 2 types of audio adapters. Some have a sound card (DAC / Digital to Analog Converter) integrated and the other use the DAC from the smartphone via special pins in the USB port. The latter is not supported by every device.
Adapters with integrated DAC will definitely work on the OnePlus 7T Pro. The manufacturer does not care here.
The adapter from OnePlus will definitely work and is not expensive at 8 euro. If you ordered your 7T Pro at oneplus.com you can still add the adapter to the cart.
When Apple or Huawei adapter, I have no evidence for or found against. At Amazon you will also find something.
