Microsoft has released Iphone and Ipad application for OWA for Office 365
To know more information about the application, check the link
How to configure Online Exchange: (Please do the Online Exchange Powershell)
To Enable for all users
Get-CasMailbox | set-casmailbox -owafordevicesenabled $True
To Disable for all users
Get-CasMailbox | set-casmailbox -owafordevicesenabled $False
Download the Iphone App here – https://itunes.apple.com/us/app/owa-for-iphone/id659503543
Download the Ipad App here – https://itunes.apple.com/us/app/owa-for-ipad/id659524331
You device should match the following requirements
iPhone 4 or higher
iPad 2 or higher.
iOS 6 or higher.
Please Note: Make sure you are on Office 365 latest version which means you should be on wave 15