There isn't actually an option to do this per say, its more an element of the camera angle. You see, when in over the shoulder view, you will find the controls different from the overhead strategic view.
Basically for Mouse and Keyboard, use the OTS Camera view, for mainly mouse controls, use the other two camera angles. You can change these in the options and by using the F1-F3 Keys in game as well I think, though check your controls menu to be certain.
Hope that helped.
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