You have configured OneStepCheckout, added products to the cart and filled out credit card information and the order submit button just won't work.
Reason behind that issue may be the fact that Terms & Conditions is not configured on your site.
Overview Terms & Conditions settings from System > Configuration > Sales > OneStepCheckout and read the RED note there that explains how to configure this part . Note that you can only use either old settings or Magento terms and conditions at the time. Using OneStepCheckout built in Terms & Conditions is deprecated and using Magento built in Terms & Conditions is suggested.