What is the difference between osteosarcoma and ewing's sarcoma?

Different cancers. These are two different types of cancer that occur in bones of young people. The locations in the body where they usually occur are different. Osteosarcoma is usually at knee or shoulder while ewing's is in thigh bone or pelvis. The chemotherapy used is different. Ewing's, unlike osteosarcoma, responds very well to radiation. Overall survival and metastasis rate is about same for both.
Sub types. Both are sarcomas occurring mostly in young adults and children mainly. Osteosarcomas always start in the bone and are more common; ewings type sarcomas occur in the bone and other soft tissue sites also. Ewings often spread to other areas unlike osteosarcoma.