To set it up, go to Settings > Background Sync in OmniFocus. When you’re in a location you want to trigger a sync—say, at home, the office, or your favorite coffee shop—tap the Add Current Location button. From now on, OmniFocus will automatically sync when you enter or leave this location.
As of this update, version 1.16, it is not possible to add locations to Background Sync unless you are in said location.