What happens when osteosarcoma spreads to a lung? Is the end near?

Not necessarily. If the number of lung nodules is small and they can be removed surgically, long term survival is possible, about 20-30 percent chance. But nobody can predict if any individual patient will die or when he or she will die. Doctors know statistics, but cannot predict individual outcome. Best thing to do is to follow the doctor's recommendations to maximize chances of survival and hope for the best.