You have configured OneStepCheckout, added products to the cart and filled out credit card information and the order submit button just won't work.

VERSION 4.*

• Open browser dev tools console and check the console tab for javascript errors. Clean One Step Checkout package should not throw any javascript errors, therefore troubleshoot the javascript if any errors are thrown and solve them.
• 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.

• Check dev tools console for javascript errors. Unmodified One Step Checkout package should not throw any javascript errors, therefore please have a critical look and resolve if they are thrown by foreign code that is loaded into One Step Checkout page.
• If you are using any javascript or html minifiers, turn them off and test. Some minifiers corrupt code and make it throw errors.