Workflowy has a variety of apps that will work when offline, and then sync when back online.
Our Desktop apps for Windows, MacOS and Linux work as a total offline solution...
Of course, our Android and iOS apps are also set up for offline use. Both will sync when your internet connection is online again.
Please note, if you are offline and make edits to an item, but also make newer edits to the same item from another online device, the edits that you had made when offline will unfortunately over-write the newer edits. We plan to improve this in the future.