I've been at this for a while. My issue is with using my OnePlus in order to run apps over ADB. As expected, when I connect my OnePlus without USB Debugging enabled, it is connected as a "Portable Device" in my Device Manager. However, when I turn on USB Debugging, the device "disconnects" from the computer altogether (I can't even access the files on it).
I've tried numerous tutorials and driver installers, all unsuccessful. I have the latest Android Studio along with the latest ADB drivers. If anyone could point me in the right direction, that would be much appreciated!
Update: The (ancient) Samsung Galaxy SII works properly when connected to my computer. I can only assume this is an issue with my OnePlus.
I managed to resolve this problem, albeit after many hours, by following these really helpful instructions. I also had to put device into PTP mode, as mentioned by @mattrick in an earlier answer.
If you don't have Android SDK installed, please install it first.
C:\Program Files (x86)\Android\android-sdk\extras\google\usb_driver
Hope this helps! Here's my source: visualgdb.com