Can Craft be used on same site as other eCommerce Solutions
You shouldn't have any issues running multiple applications on the same host but it would be smarter to decouple the different applications and build a bridge between them.

I've never worked with OSCommerce or OpenCart so I can't speak to the specifics of what's involved in doing that but if your high-level goal is to run Craft on the same server as the other application(s), that should be fine.