You need to add the sheetname followed by
!
to each cell reference in the formula, for example:
((Excel.Range)oExcel.Cells[9, 3]).Formula = "=SUM(Sheet1!C4,Sheet1!C7)";
Note that if the range is a consecutive set of cells you only need one reference to the sheet thus:
((Excel.Range)oExcel.Cells[9, 3]).Formula = "=SUM(Sheet1!C4:C7)";