Key Inclusion Criteria:

          -  Provision of Informed Consent

          -  Male and female patients who are ≥6 months to <18 years of age at consent

          -  Pathologically confirmed relapsed or refractory solid or primary CNS tumours
             (excluding lymphoid malignancies), with a HRR deficiency/gene mutation, and for whom
             there are no standard treatment options. Eligible patients may include but not be
             limited to those with osteosarcoma, rhabdomyosarcoma, non rhabdomyosarcoma soft tissue
             sarcoma, Ewing Sarcoma, neuroblastoma, medulloblastoma and glioma

          -  For dose finding phase only: recruitment will be open to all patients with HRR
             deficiency, based on a local test. For the signal identification phase: recruitment
             will be open only to patients with documented evidence of a deleterious or suspected
             deleterious germline or tumour HRR gene mutation that meets the AZ HRR rules

          -  A formalin fixed, paraffin embedded (FFPE) tumour sample from the primary cancer (all
             patients) suitable for central HRR testing and a blood sample (patients ≥2 years old)
             for central germline BRCA testing must be provided for each patient

          -  For all non-neuroblastoma tumours, patients must have at least 1 radiographical
             assessable lesion (measurable and/or non-measurable). For neuroblastoma tumours,
             patients must have radiographical assessable disease with at least 1 lesion
             (measurable and/or non measurable) OR disease evidenced by uptake of
             meta-iodobenzylguanidine- (MIBG) or fluorodeoxyglucose positron emission tomography
             (FDG-PET) scans

          -  Adequate performance status, organ, and marrow function and adequate weight to obtain
             blood samples for both safety laboratory assessments and PK analysis.

          -  Ability to swallow tablets

        Key Exclusion Criteria:

          -  Patients with MDS/AML or with features suggestive of MDS/AML

          -  Patients unable to swallow orally administered medication

          -  Unresolved toxicity from previous anticancer therapy

          -  Unstable or untreated CNS disease (i.e., symptomatic uncontrolled brain metastases or
             untreated spinal cord compression)

          -  Previous treatment with a PARP inhibitor, including olaparib

          -  Receipt of any radiotherapy for cancer treatment (except for palliative reasons)
             within 30 days prior to first dose of study treatment or receipt of last dose of an
             approved (marketed) anticancer therapy (chemotherapy, targeted therapy, biologic
             therapy, monoclonal antibodies, etc) within 21 days prior to the first dose of study
             treatment

          -  Concomitant use of known strong or moderate CYP3A inhibitors or concomitant use of
             known strong or moderate CYP3A inducers

          -  Whole blood transfusions in the last 120 days prior to screening (packed red blood
             cells and platelet transfusions are acceptable)