Option Explicit

Private WithEvents App As Application

Private Sub Workbook_Open()
    Set App = Application
End Sub

Private Sub App_WorkbookOpen(ByVal Wb As Workbook)
    'Statement to show that it works
    MsgBox "Opened Workbook: " & Wb.Name
    'Statement to autofit columns
    ActiveSheet.UsedRange.Columns.AutoFit
End Sub