The official documentation for OpenOffice BASIC is located at:
https://wiki.openoffice.org/wiki/MediaWiki:Collections/BASIC_Guide
An abridged summary of the language (PDF).
Notes on customising OO to make it easier to use the programming environment (PDF).
An alternative but very long guide to the official documentation is a guide written by
Andrew Pitonyak
under the title "OpenOffice.org Macro document". It is
available at:
http://www.pitonyak.org/oo.php
Note that there are other guides for OO Basic on the same webpage.