There is no way to perfectly imitate any phone. Starting the Nexus 5 emulator will bring you a βdeviceβ with some correct values, such as the screen coordinates or the system version of Android, but it wonβt make you sure that if it works on the Nexus 5 emulator, it will work on a real device.
As I know, OnePlus uses Cyanogen OS (so this is not the same as you would find on Nexuses or Samsung devices), so one of the possibilities is to find a cheap version of the phone with this Android user system, such as Willey Fox brand devices.
In addition, test your application on the Genymotion emulator - it also uses a special Android system based on CyanogenMod, but, as you already know, the best way is to test it on a real device.
Hope this helps