QuickBooks Online currently does not export to QuickBooks for Mac directly. See related article Export limitations for more information on export limitations.
To transition from QuickBooks Online to QuickBooks for Mac, you will need to export the file first to QuickBooks for Windows version then convert to QuickBooks for Mac. The Windows desktop version of QuickBooks software has to be the same year or older than your Mac version. For example, to export to Pro desktop from Mac 2010, you will need to have Pro 2010 for Windows, or older. If you have access to a QuickBooks for Windows that has the same year or older than your Mac version, see Visual guide to export.
If you don't have access to a QuickBooks for Windows version that has the same year or older than your Mac version, consider asking your accountant for a copy of the file. Otherwise contact QuickBooks Online Customer Care at 1-800-488-7330 during regular business hours for other options.