John Jordan,
I am hoping you are on the latest version of Office 365, here is an article to check that to be sure about it: http://office.microsoft.com/en-us/help/am-i-using-office-365-after-the-service-upgrade-HA103982331.aspx. OWA App uses autodiscover to set up your account on the device. Have you published required DNS records for autodiscover to work properly for your users? If you have done that then please run the "Outlook Autodiscover" test under "Office 365" tab at exrca.com. Use the same user's credentials that you're using on the iOS device in OWA App. If the test succeeds then there may be some other issue, and we can help you with that. You can open a Support Incident thru MOP or post your issue in Office 365 'Mobile Access' Forum (http://community.office365.com/en-us/forums/173.aspx). A support engineer will then work with you to help resolve your issue. If the "Outlook Autodiscover" test fails, then you will need to take the steps outlined in these articles to setup autodiscover properly: http://help.outlook.com/en-us/cc950655.aspx & http://support.microsoft.com/kb/2404385 (see "Method 2") HTH.