Workbook_open not firing due to compile error in class module (missing library reference)

Hi all,
after hours of research in the web I finally need to ask here for your help: Following problem: I try to add a reference (Outlook library) in the Private Sub Workbook_Open() event (yes, it's in ThisWorkbook) since I need early binding for getting Outlook events later in different places. Once I open my workbook I get a compile error "user-defined type not defined" in my class module here --> Private olApp As New Outlook.Application. I understand this since the Outlook reference is not yet set and this is what I exactly try to do in the Workbook_Open event. How does Excel work here? Does it first try to compile the project BEFORE it executes code in the Workbook_Open event (enableEvents is set to True)?
Once I run the Sub Workbook_open manually, the reference is set without any problems.
Thanks in advance for your help.

Trying to add reference for MS Outlook Object Library when Workbook opens:

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Here the Class Module "ClsAppoint" in which the compile error occurs once I open the workbook (red line):

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I found the problem
All buttons and checkboxes in the workbook were Form Controls and different macros were assigned to those controls. I found out that if I remove all macro assignments and reopen the workbook, the compile error disappeared and the reference was set successfully. I replaced all Form Controls by Active X Controls and everything works like a charm. Don't ask me why it didn't work with Form Controls. I'm just glad that it's working now.