A complete offline is available with Odata SDK SMP 3.0 now. SMP 3.0 Odata SDK is supported with Android and IOS, Windows 8 may be supported in the future. But understand that Odata is open standard and it allows you to connect to any mobile OS. In the Odata SDK you will have APIs for connection/on board a user, offline etc, but if you are not using the Odata SDK you can do it manually too, ie you can develop win 8 apps even now.
If you are planning to develop offline app using Odata with SMP lower versions like 2.3 or 2.2 you will not get APIs that support offline, but by putting some effort you can create a offline app too. You need to make use of the local DB of the device. ex, Android has SQLite DB you need to write code to create a table to store the data you are getting through Odata call. When the user is trying to do an operation you can write code to check whether the device is online or not and save the data locally in device if its offline and can be updated to backend when the device is online. The advantages of offline APIs is that you don't want to bother about the data storage and its management when the app runs.
Examples are provided in the previous comments.
Midhun VP
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