Olaparib With Ceralasertib in Recurrent Osteosarcoma

This study is being done in order to evaluate the effectiveness of using two drugs (olaparib and ceralasertib) to treat patients with osteosarcoma that has not responded to treatment or has come back after treatment The names of the study drugs involved in this study are: * Olaparib * Ceralasertib

Inclusion Criteria: * Provision of informed consent prior to any study specific procedures. * Age \> 12 years and ≤ 40 years * Weight \> 40 kg * Lansky/Karnofsky performance status ≥ 60% for participants ≥16 years of age and Lansky ≥ 60% for participants \10 mg of prednisolone per day or an equivalent dose of other corticosteroid. If on corticosteroids, the patient should be receiving a stable dose of corticosteroids, started at least 4 weeks prior to treatment. * Female patients who are pregnant or breast-feeding. * Male or female patients of reproductive potential who are not employing an effective method of birth control. * Spinal cord compression or brain metastases unless treated, asymptomatic and stable and not requiring steroids for at least 4 weeks prior to start of study treatment.