The Opera developers work in highly unprofessional manner:

Look at Opera 20 so called "help": how to import bookmarks?
or

type in browser opera:flags , and
serach "import"
--
The developers wrote:

Enable or disable profile import from Opera 12 or other Presto-based Opera versions. Presto's profile location can be given explicitly by passing '--presto-small-prefs-dir=ROAMING_PROFILE_DIR' and '--presto-large-prefs-dir=LOCAL_PROFILE_DIR' to the command line.

This recommndatio of the developmentteam is wrong for Windows 7/8.1.