So, with css buttons disabled the behaviour is as expected. The submit button looks like this:

Code:
<img src="includes/templates/template_default/buttons/english/button_submit.gif" alt="Apply" title=" Apply " width="51" height="15" name="apply_gv" class="opc-cc-submit">
So, I am guessing that the class is what triggers the javascript. But without dismantling the javascript I am not 100% on that. It feels like a glitch in html_output.php rather than with OPC.

Moving forward with that theory:
Changing line 395 of html_ouptut.php to something like the following works for me but I have no idea if that has consequences beyond the scope here.

Code:
$css_button = '<span class="'.$sec_class.'" ' . $tooltip . $parameters . '>&nbsp;' . $text . '&nbsp;</span>';