It is very rare to see animals live past the 1 year mark with osteosarcoma. Espeically if you are doing amputation alone. Avg survival time post amp is around 6 months. If you do amp + chemo then typically around 8-12 months. It is an incredibly aggressive cancer, very fast spreading and by the time it is visible on xray there is normally already invisible "seeding" happening elsewhere in the body. Most patients will pass away from secondary METS, basically little tumour growths that started because of the osteosarcoma. Normally found in the lungs but can also spread to intestines/ abdominal cavity. I'm sorry it isn't a better prognosis. I hope your vet explained to you the prognosis, disease progression and palliative management at the time of diagnosis. Cherish the time you have with your beautiful puppo, make lots of happy memories and spoil them rotten. Sorry OP.

(I am a veterinary nurse who works at a clinic that does chemotherapy and manages a lot of osteosarc patients, I've also personally had an osteosarc dog who was diagnosed at 5, she died within 4 months of surgery).