You must not have several 3rd party checkout or 3rd party onestepcheckout modules installed on your Magento at the same time.

When installing One Step Checkout, please uninstall beforehand all other 3rd party checkout modules, paying extra attention to remove filenames with matching names like onestepcheckout.xml or folders /onestepcheckout in your theme folders.

We at onestepcheckout.com support do not have the knowhow about other companies modules and are not able to proceed with above mentioned uninstall. Please contact the module providers support for complete uninstall instructions or have them uninstall.