Olaparib With Ceralasertib in Recurrent Osteosarcoma

This is a single arm, phase 2, open-label clinical trial to evaluate the use of olaparib in combination with ceralasertib in 2 cohorts of patients aged 12-40 with recurrent osteosarcoma. The research study procedures include screening for eligibility, study treatment, evaluations and follow-up visits. * Cohort 1: Participants with unresectable osteosarcoma (unable to remove with surgery) * Cohort 2: Participants with lung only resectable osteosarcoma (able to remove with surgery) Participants will be given a drug diary to document information about the study treatment and study calender with information about what to expect during and between study visits. The names of the study drugs involved in this study are: * Olaparib * Ceralasertib Each treatment cycle lasts 28 days and participants will receive study treatment up to 24 cycles (2 years). It is expected that about 63 people will take part in this research study. * The study is looking to test: whether olaparib and ceralasertib given together are effective in patients with recurrent or refractory osteosarcoma. * how safe and how well tolerated olaparib and ceralasertib are when given together in patients with recurrent or refractory osteosarcoma. * markers in the blood and in tumor tissue to see if there are certain features of the tumor that may indicate this combination of drugs is effective or not effective. The U.S. Food and Drug Administration (FDA) has not approved Ceralasertib as a treatment for any disease. This is the first time that Ceralasertib will be given to children. The U.S. Food and Drug Administration (FDA) has not approved olaparib for recurrent osteosarcoma but it has been approved for other uses.