Synchronized Data across Devices

With the rise of multi-devices offering (e.g.: Todoist, Wunderlist, Evernote, etc), people will surely face technical challenges of synchronizing across devices (Web, Tablet, Phones, XBox, Native Client) including offline modification.

I'm wondering if Optimistic Locking is enough or perhaps it requires Operational Transformation. I'm aware that OT is probably a better use-case for collaborative editing but not everything is collaborative editing.

For example: editing a todo item via offline devices that will eventually become online. Say I have 2 devices (tablet and phone), both offline somehow and I made modification to the same object using this device...

What are the approaches to solve this problem?