is it possible that creating a shopping cart we can use codeigniter as frond end and oscommerce as backend
i have doubt that when you this is there any problems with the htaccess
Yes, it is possible.
You can decouple the front-end from OSCommerce. You can then just get the data needed for the frontend from the DB. It's gonna be the point-of-interaction for both ends, just be sure you study well the DB structure of OSCommerce. But you should be aware that this will effect functionalities that relies on data gathered from the front-end, like, analytics.
There will be problem with the HTAccess if it's not configured properly. Are you planning to pretify your URL for CI?