All photos and videos will be uploaded at their original sizes when you're on a Wi‑Fi network. If you want to use your mobile network, tap the three horizontal bars in the upper-left corner, tap Settings, tap Camera Backup, and then turn on Use Mobile Network.
To set your device to upload pictures even when the OneDrive app isn't the active app, tap Upload in Background.
The camera roll on your device isn't kept in sync with the Camera roll folder in OneDrive. After photos and videos finish uploading, you can delete them from your device, and the copies in OneDrive won't be affected.
Source:
http://windows.microsoft.com/en-us/onedrive/ios-faq