OverscriptBox[x, y] represents in input and output.
Inside \( ... \) OverscriptBox[x, y] can be input as x \& y.
In a notebook an OverscriptBox can be created using 7 or &. moves out of the overscript position.
In StandardForm and InputForm, OverscriptBox[x, y] is interpreted on input as Overscript[x, y].
The following special interpretations are made:
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.
In StandardForm, explicit OverscriptBox objects are output literally. You can use DisplayForm to see the display form of such objects.
See The Mathematica Book: Section 2.8.10.
See also: UnderscriptBox, UnderoverscriptBox, SuperscriptBox, GridBox, FractionBox, ScriptSizeMultipliers.