Yes, you should be able to use Outlook XP with Exchange 5.5.
However, one thing to be aware of is that if your Outlook XP is set to use other parts of Office XP, you may experience other problems. For instance... if you have Tools --> Options --> Mail Format --> and either one of the "Use MS Word to edit.." is checked, then other users of the document may have some problems reading it if they are using Office(s) older than Office XP.
However, you should not have any problems accessing OWA. This is done through your browser, not through Office.