Sum Function Fails: Automate The Columns

I have a spreadsheet that I am trying to automate, one of the tasks is to sum some columns namely columns O to T. Column B contains the exact number of entries in the columns O to T, therefore I thought that this would be a good counter. The Spreadsheet varies in number of entries every time. In this instance the rows are in total 200 inclusive a header row. ie r below is = to 200. I have tried to record a macro and from that I have tried to replace the pertinent values.

Sub Sel_Sum ()
Dim r As Integer
Dim s As Integer
Dim t As Integer
Dim U As Integer
Range("B1", Range("B65536").End(xlUp)).Select
r = Selection.Rows.Count
s = (r - 1) * -1
t = r + 1
U = -1
Range("O" & t, "T" & t).Select
Selection.FormulaR1C1 = "=SUM(R" & [s] & "C:R" & [u] & "C)" 'fails here
Selection.NumberFormat = "#,##0.00_ ;[Red]-#,##0.00 "
End Sub