The OWA apps for iPhone, iPad, and Android mobile devices will no longer work from tomorrow (15 May 2018) onwards. If you currently use this app on any of your mobile devices, you need to download and install the Outlook for iOS or Outlook for Android app instead.
The Outlook apps offer enhanced email and calendar features, and most of the functionality currently found in the OWA apps. A few features such as the ability to access shared mailboxes will not be available until later in the year.
More information can be found in Microsoft’s support.office.com article.