If you have a subscription that allows map downloads, then you can download your maps for offline use. This allows you to download maps and use them away from an internet connection.
You can either elect to download a complete product, or expand the product and choose which maps you wish to download. For more details on this, please see Working with the My Maps screen. Please note that some map data sets are very large, and therefore downloading a complete set can take a considerable amount of time and use up considerable storage on your computer.
If you are downloading a lot of maps, please leave the My Maps window open, else iOS may stop the map downloads if OMN is not the foreground application.
Checking your maps are ready for Offline use.
The best place to check, is in the My Maps screen, where each map lists its state - Installed, Partial or Downloadable. If the map is Installed, then it is on your device.
If you put your phone into 'Aeroplane Mode', this turns off all wifi and cellular connectivity. This is a good way to simulate being on a hill with no internet connectivity. You should be able to view your maps easily.
If you have any issues, please see: Repairing Map Data
Sizes of data
To see how much space the maps take up, please see this link: