If you were previously using another WebDAV provider (or syncing via Bonjour), you can switch to the free Omni Sync Server by following these steps. If you don’t already have an account, you’ll want to sign up for one on this page.
Step 1: Sync all of your devices
Start by syncing all your devices with your old settings/server. You want all your devices to be in sync before you migrate. If you no longer have access to your old sync location, that’s okay too; skip to Step 2 and and use the device which is most up-to-date instead of your Mac.
Once all your devices are in sync, sync one of your Macs one more time with the old server (on addition to the sync you did in the previous step). You’re going to use your Mac as the basis for the copy of your database that will live on the sync server.
(If you’re not using OmniFocus for Mac, skip to Step 2 and carry out the following steps on one of your iOS devices instead of a Mac.)
On the same Mac you just synced, choose File ▸ Export, and save your database in the OmniFocus Document file format. This gives you a fallback point if anything unexpected happens during this process.
Step 2: Sync one device to the Omni Sync Server
Open the OmniFocus sync settings, then select Omni Sync Server as your sync method. Enter the account name you specified when creating your new Omni Sync Server account. Use the Sync Now button to initiate your first sync with our server, and provide your Omni Sync Server password when prompted.
Step 3: Sync your other devices
Head to the sync settings on your other devices, and choose the same Omni Sync Server option (using the same account name), then sync. On Mac, when asked to replace your database click Sync to proceed. On iOS, you may receive a message indicating that there’s already data on the server. Choose the Keep Sync Database option to connect this OmniFocus installation to that existing server data.