If you want to make a simple change to the checkout template, like for example change some text, add some images or anything you might want, the procedure is quite easy. There is a single main template of OneStepCheckout called '''checkout.phtml'''.
With Magento 1.4, Magento Enterprise and Magento Professional, the default location is: * app/design/frontend/base/default/template/onestepcheckout/checkout.phtml
With Magento 1.3 the default location is: * app/design/frontend/default/default/template/onestepcheckout/checkout.phtml
'''NB:''' You should copy this file from base/default to '''yourpackage/yourtheme''' (usually something like default/customtheme) in order to not touch the core OneStepCheckout template file, but rather have a copy of it. Because if you touch the default one, you will be affected when [[Upgrade_guide|upgrading OneStepCheckout]].