Currently, if you wish to use OOUI in a MediaWiki extension, you have to set up the library to use the MediaWiki theme and to output text with the appropriate directionality:
OOUI\Theme::setSingleton( new OOUI\MediaWikiTheme() ); OOUI\Element::setDefaultDir( $wgContLang->getDir() );
Setting up OOUI should not be left up to individual extensions. It should be done centrally, in MediaWiki-Core.