In settings->Data you will find a section on storage.

Android specifications state that all devices must have an Internal and External storage. Historically external storage was as its name suggests separate storage. 

However, on most modern phones, External is now just part of the main storage supplied with the device.

OMN will attempt to detect the storage locations where it can save data. If you tap on the combo list, you will have Internal, External and Custom listed. You may also have SD Card listed if OMN has found one.

If you change to Internal, External or SD Card, you can then tap on Move Maps, and your data will be moved.

Alternatively, if the path you wish to use is not automatically listed, then you can select Custom. You can then input the path you wish to use and tap on the ‘Test Path’ button. If that test is successful the Move Maps button will become enabled, and you can move your data.

Note: You can’t move the OMN app from the internal storage using the Android Settings screen – this would just move the app which is about 100MB, and leave the map data on the internal storage. We therefore disabled this to stop confusion.