Loading icon spinning

From OneStepCheckoutWiki

If the loading icon keeps spinning and never disappears, it is because OneStepCheckout is not accessed through the correct URL. If your Magento store is configured to use SSL (HTTPS), you need to access OneStepCheckout on a HTTPS URL, and not the HTTP one.

So if you are getting a forever loading spinning icon on:

http://www.yourdomain.com/onestepcheckout

Please navigate your browser to:

https://www.yourdomain.com/onestepcheckout

And it should work properly.