Today I wanted to learn Flutter. I read through the docs and found out that I can run my app on my real device (Oppo F5 in this case).
I do have enabled USB Debugging. MTP works fine; I can transfer files as usual).
When I enable USB Debugging, a driver install window pops up (I'm using Windows 7 x64). It tries to detect ADB Interface from Windows Update and fails; then tries to install from the preconfigured driver folders, then fails.
So, after a bit of Googling, I found out a solution. I just needed to install a thing called "OPPO Preloader Driver". Thank you everyone who participated in this thread!