In this example, the rectangular range of each cell of the active sheet is selected. In addition, it uses indexed Range parameters to get range points. In addition, AddEx () (and most of the methods in Interop.Excel) uses the default parameters, so you do not need to use System.Reflection.Missing.
Excel.Range oRng1 = oSheet.Range["A1"];
Excel.Range oRng2 = oSheet.Range["A1"].End[Excel.XlDirection.xlToRight]
.End[Excel.XlDirection.xlDown];
oRng = oSheet.Range[oRng1, oRng2];
oRng.Select();
oRng.Worksheet.ListObjects.AddEx(
SourceType: Excel.XlListObjectSourceType.xlSrcRange,
Source: oRng,
XlListObjectHasHeaders: Excel.XlYesNoGuess.xlYes);