I have my phone backing up my pictures to OneDrive. However, since I'm using Windows 10 Mobile, I've been resetting my phone fairly frequently. When I do this I lose all my pictures on my phone (of course). Since the Photos app shows content from OneDrive I can still see all my pictures, but because they're not local I have severe issues viewing them when I don't have good service. So I want to sync them to the phone for offline viewing. I would just download all the pictures and copy them over, but this seems to create issues with duplicate uploads, etc. On desktop you have the option to sync OneDrive folders offline, which works nicely. But is there any way to get all your pictures offline on mobile?
I would also love for this functionality to exist in WM10 and have voted for it on the Feedback app. I hate to be that guy, but as far as I know and at least on the current version of the OS the only way to make this happen is to select all pics you want from onedrive and doenload it to the pics folder of your phone that you choose manually. Photos now has the option to chose what folder from onedrive it should look for and if properly organised the photos in folder you shouldn't have duplicates. I know it's not ideal and I really hope they implement this kind of functionality in a future update.