Oscommerce V2.4 will be released in the near future, it includes an improved and modernized code base, extended modularity and responsive design.

For those that does can’t or won’t want to wait for osCommerce 2.4, there is available a community driven responsive version of osCommerce 2.3.4 available now as well as an extension for making an existing osCommerce 2.3+ based shop responsive.

Resources:
– Responsive HTML 5 osCommerce
– Responsive osCommerce 2.3.4 with Bootstrap 3