OverscriptBox is the low-level representation of Overscript. Except for low-level notebook expression manipulation, OverscriptBox should not need to be used directly.
Inside \(...\)OverscriptBox[x, y] can be input as x\&y.
In a notebook, an OverscriptBox can be created using Ctrl+7 or Ctrl+&. Ctrl+Space moves out of the overscript position.
For these special cases special input forms such as x\&_ can be used.
The baseline of OverscriptBox[x, y] is taken to be the baseline of x.
OverscriptBox[x, y] is usually output with y in a smaller font than x.
With the option setting LimitsPositioning->True, y is placed in an overscript position when the whole OverscriptBox is displayed large, and in a superscript position when it is displayed smaller.