If your Outlook is configured with the setting "Use Cached Exchange Mode" selected this file can become out of sync. This behavior occurs with Outlook on all platforms and is not specific to Crossover.

If Outlook tells you to delete your Outlook.ost file and restart the program you will need to delete the file at

Linux: /home/(Your Account)/.cxoffice/(Bottle Name)/drive_c/windows/profiles/crossover/Local Settings/Application Data/Microsoft/Outlook/outlook.ost
Mac: (Your Home Folder)/MailBox/outlook.ost