If you are familiar with Magento layout XML files and templates, the OneStepCheckout design will be easy.
By default, the checkout template is located in '''app/design/frontend/base/default/template/onestepcheckout/checkout.phtml'''
In the same folder, you will see other templates for the checkout, such as payment and shipping method templates, summary and others.
If you ned to make changes to the layout, the default file is in '''app/design/frontend/base/default/layout/onestepcheckout.xml'''
'''PS:''' Remember that when making changes to OneStepCheckout files, you should copy the templates and XML files you change from base/default to yourpackage/yourtheme (usually default/yourtheme or similar)
Also, cache must be turned off while making these changes.