Google has added offline support for Slides app in Google Docs. Docs already has the same functionality.
Users can now create, edit and present their Google Docs hosted presentations without requiring an active internet connection.
You can now create, edit, comment and present without an internet connection—just like you can with Docs. Put the finishing touches on your presentation mid-flight, and head to your next big presentation without worrying about a flaky internet connection.
The company added that same functionality is coming soon to spreadsheets component of Google Docs.