If you already have the outline applied, you can protect the worksheet in code
(auto_open/workbook_open??).
Option Explicit
Sub auto_open()
With Worksheets("sheet1")
.Protect Password:="hi", userinterfaceonly:=True
.EnableOutlining = True
End With
End Sub
It needs to be reset each time you open the workbook. (excel doesn't remember
it after closing the workbook.)