OneStepCheckout version 3 and later will automatically add the comment field to the order detail page in the administration interface. Previous versions must follow the old [http://www.onestepcheckout.com/skin/frontend/default/onestepcheckout/media/onestepcheckout-installation-guide.pdf installation guide for versions 2.2 and below].
You can also add the customer comment to the order email, as well as generated PDF's (a bit more complicated). Here is how to add it to email templates:
For using this variable, please make sure you have set the setting "Merge comments to default comments collection" to NO in System>Configuration>Sales>OneStepCheckout>
NB! If comment or feedback value does not show in Magento admin, or you have added variables to email or pdf and the value does not show: