Sub O_Negative()
Dim LstRow As String
Dim Rng As Range
Dim SubRng As Range
LstRow = Cells(Rows.Count, "L").End(xlUp).Row
Set Rng = Range("T2:T" & LstRow)
For Each SubRng In Rng
If SubRng = "O" Then
Range("L" & SubRng.Row) = Range("L" & SubRng.Row) * -1
End If
Next SubRng
End Sub