Hi Ben,
The Khan Academy app on iOS in the following link
https://itunes.apple.com/app/apple-store/id469863705?mt=8
Or Android based app
https://play.google.com/store/apps/details?id=org.khanacademy.android

Allows for the ability to download videos to your mobile device and will have the ability to work offline. The only thing is depending the space on your device you may limit the videos that can be played offline.

I hope this helps if you have any other questions I will be happy to answer them.