Private Sub Worksheet_Change(ByVal Target As Range)
Dim xrg, s$
If Intersect(Range("a1:j1"), Target) Is Nothing Then Exit Sub
Set xrg = Intersect(Range("a1:j1"), Target)(1, 1)
s = LCase(xrg)
Application.EnableEvents = False
Range("a1:j1") = "X"
If s = "o" Then Cells(1, xrg.Column) = "O"
Application.EnableEvents = True
End Sub