Salut Jefekoi,
Sans formule : VBA, je ne vois pas d'autre solution.
Private Sub Worksheet_Change(ByVal Target As Range)
'
Application.EnableEvents = False
'
If Not Intersect(Target, [F5]) Is Nothing Then _
[F15] = IIf([F5] = "X", "OK", ""): _
[F15].Interior.ColorIndex = IIf([F5] = "X", 3, xlColorIndexNone)
'
Application.EnableEvents = True
'
End Sub
A+