where to install with oscommerce?

[eluser]pbflash[/eluser]
You can run CI alongside OSC. You need to decide if OSC or CI will be used as the main part of your site. Whatever will be the main part should be installed in the root folder and the other in a sub-folder. I'm not sure how creating an addon with CI would work as OSC already has addon functionality and your addon would need to be coded to work within their coding requirements.