This is how the OAuth authentication form looks with JS disabled (ie. falling back to a PHP-generated HTML form - whether with plain HtmlForm or OOUIHtmlForm as in 201230 does not make much difference):
This is compatible with how other MediaWiki forms/interfaces look and with the style guide for buttons.
This is how it looks with JS enabled, using the OOUI MessageDialog (via 201230):
The button layout is completely alien from the rest of MediaWiki, even though the classes generated on the buttons are the same.