To setup OAuth to allow you to migrate users into multiple domains, you will need to configure OAuth as follows:
In the Google Apps control panel, go to the 'Advanced tools' tab.
In the 'Authentication' section, click 'Manage OAuth domain key'.
Make sure that the 'Allow access to all APIs' box *isn't* ticked.
Click the 'Advanced tools' tab.
In the 'Authentication' section, click 'Manage third party OAuth Client access'.
Under 'Client Name', type the name of your consumer key. This is usually the name of your primary Google Apps domain.
Under 'One or More API Scopes', type the following text: https://apps-apis.google.com/a/feeds/migration/,https://www.google.com/calendar/feeds/,https://www.google.com/m8/feeds/,https://apps-apis.google.com/a/feeds/user/#readonly