• Patients must provide written informed consent prior to performance of study-specific procedures or assessments.

• ARID1A Subgroup (N = 39):

∙ Histologically confirmed locally advanced or metastatic solid tumor malignancy with progression on at least one prior systemic therapy, including one of the following tumor types:

⁃ Renal cell carcinoma with predominant clear cell histology (Cohort A)

• Urothelial carcinoma (Cohort B)

• All pancreatic cancers (Cohort C)

• Other solid tumors excluding clear cell ovarian cancer and endometrial cancer (Cohort D)

• Endometrial and ovarian cancer (Cohort E)

‣ Formalin-fixed paraffin embedded tumor tissue evaluable for BAF250a expression by ARID1A immunohistochemistry. Primary or metastatic tumor tissue is permissible. Patients without evaluable archival tissue may undergo optional tumor biopsy during Screening if other eligibility criteria have been met

‣ Measurable disease by RECIST 1.1

• ATM Loss Subgroup (N = 20):

∙ Histologically confirmed locally advanced or metastatic solid tumor malignancy with progression on at least one prior systemic therapy, including one of the following tumor types:

⁃ Metastatic castration resistant prostate cancer (N = 10).

∙ Patients may have evaluable or measurable disease by RECIST 1.1 criteria.

‣ Prior treatment with at least one androgen signaling inhibitor (e.g. abiraterone, enzalutamide, apalutamide, darolutamide).

‣ Patients will be required to maintain castrate levels of testosterone during study treatment with use of Luteinizing hormone-releasing hormone (LHRH) analog (except for patients with history of bilateral orchiectomy).

‣ Progression by PCWG3 criteria at study entry

• All other solid tumor malignancies (N = 10). Patients are required to have measurable soft tissue disease by RECIST 1.1 criteria.

‣ Archival tumor tissue evaluable for ATM expression by immunohistochemistry (IHC)

‣ Evidence of ATM loss by either pathogenic ATM mutation in Chemiluminescent immunoassay (CLIA)-approved assay and/or loss of ATM expression by IHC (Ventana Ab). An interim analysis will be performed after 10 patients are enrolled. If less than 50% of tumors have absence of ATM expression by IHC, subsequent enrollment of the remaining 10 patients will be required to have evidence of both ATM mutation and loss of ATM expression (\< 5% of tumor cells expressing ATM) using CLIA-certified IHC test (Ventana).

• Endometrial Cancer Cohort (N = 30):

∙ Histologically confirmed endometrial cancer

∙ o A minimum of 15 patients must have the presence of pathogenic ARID1A alteration on CLIA-approved next-generation sequencing panel without evidence of microsatellite instability defined by next-generation sequencing and/or presence of intact mismatch repair proteins by immunohistochemistry.

‣ Measurable disease by RECIST 1.1.

‣ Availability of archival tumor tissue for retrospective testing of BAF250a expression by IHC.

‣ Has received at least one prior line of systemic therapy for the treatment of locally advanced or metastatic disease, including progression on at least one prior line of therapy containing an immune checkpoint inhibitor that was administered for a minimum duration of 6 weeks

⁃ Must not have experienced a toxicity that led to permanent discontinuation of prior immune checkpoint inhibitor.

• All adverse events (AE) while receiving prior immune checkpoint inhibitor must have completely resolved or resolved to baseline prior to screening for this study.

• Must not have experienced a \>= Grade 3 immune related AE or an immune related neurologic or ocular AE of any grade while receiving prior immune checkpoint inhibitor. NOTE: Patients with endocrine AE of \<=Grade 2 are permitted to enroll if they are stably maintained on appropriate replacement therapy and are asymptomatic.

• Must not have required the use of additional immunosuppression other than corticosteroids for the management of an AE, not have experienced recurrence of an AE if re-challenged, and not currently require maintenance doses of \> 10 mg prednisone or equivalent per day.

‣ Body weight \>30 kg

‣ No active or prior documented autoimmune or inflammatory disorders (including inflammatory bowel disease \[e.g., colitis or Crohn's disease\], diverticulitis \[with the exception of diverticulosis\], systemic lupus erythematosus, Sarcoidosis syndrome, or Wegener syndrome \[granulomatosis with polyangiitis, Graves' disease, rheumatoid arthritis, hypophysitis, uveitis, etc\]). The following are exceptions to this criterion:

⁃ Patients with vitiligo or alopecia.

• Patients with hypothyroidism (e.g., following Hashimoto syndrome) stable on hormone replacement.

• Any chronic skin condition that does not require systemic therapy.

• Patients without active disease in the last 5 years may be included but only after consultation with the Principal Investigator.

• Patients with celiac disease controlled by diet alone.

• Evidence of clinical or radiographic progression prior to study entry (except metastatic castrate-resistant prostate cancer (mCRPC) cohort which requires progression by PCWG3 criteria).

• Age \>= 18 years at time of signing informed consent form.

• Resolution of all prior treatment-related toxicities to grade 1 severity or lower (except alopecia).

• Patients must be at least 3 weeks or 5 half-lives (whichever is shorter) from last standard or experimental non-cytotoxic therapy prior to first dose of protocol therapy. Patients must be \> 21 days from last dose of cytotoxic chemotherapy prior to C1D1. The minimum wash-out period for immunotherapy is 42 days prior to C1D1 with the following exception for the Endometrial cohort: Note: Washout from prior immunotherapy is \>=21 days to C1D1 for the Endometrial cohort.

• Radiation therapy must be completed \> 7 days prior to course 1 day 1 (C1D1) or \> 28 days prior to C1D1 for patients receiving radiation to more than 30% of bone marrow.

⁃ Adequate organ function as defined by:

∙ Hemoglobin (Hgb) \>= 9.0 g/dL in the absence of transfusion within 14 days prior to screening laboratory assessment.

‣ Platelets (Plt) count \> 100,000 x 10\^9/L.

‣ Absolute neutrophil count \> 1.5 x 10\^9/L.

‣ Estimated glomerular filtration rate (GFR) \>= 45 ml/min based on Cockcroft-Gault equation or 24 hour urine collection.

‣ Aspartate aminotransferase (AST)/alanine aminotransferase (ALT) \< 2.5 x upper limit of normal (ULN) (\< 5x ULN in patients with known liver metastases).

‣ Total bilirubin \< 1.5 x ULN (direct bilirubin \< 1.5 x ULN in patients with known Gilbert's disease or UGT1A1 homozygote).

⁃ Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1.

⁃ The effects of ceralasertib and olaparib on the developing human fetus are unknown. For this reason and because ATR and PARP inhibitors as well as other therapeutic drugs used in this trial are known to be teratogenic, women of child-bearing potential and men must agree to use 2 highly effective forms of contraception prior to study entry and for the duration of study participation. Should a woman become pregnant or suspect she is pregnant while she or her partner is participating in this study, she should inform her treating physician immediately.

• Male patients who are sexually active must be willing to use barrier contraception for the duration of the study and for 1 week after the last study drug administration, with all sexual partners. Male patients must use a condom during treatment and for 6 months after the last dose of study drug(s) when having sexual intercourse with a pregnant woman or with a woman of childbearing potential. Female partners of male patients should also use a highly effective form of contraception for 6 months after the last dose of study drug(s) if they are of childbearing potential. True abstinence for either sex is an acceptable form of contraception and must be documented as such.

∙ Women of childbearing potential must have a negative serum or urine pregnancy test within 7 days prior to C1D1 treatment. Evidence of postmenopausal status or non-child bearing status must be documented. Postmenopausal is defined as:

‣ Aged more than 50 years and amenorrheic for at least 12 months following cessation of all exogenous hormonal treatments.

⁃ Documentation of irreversible surgical sterilization by hysterectomy, bilateral oophorectomy or bilateral salpingectomy but not tubal ligation, radiation-induced oophorectomy with last menses \> 1 year ago, chemotherapy-induced menopause with \> 1 year interval since last menses

⁃ Amenorrhoeic for 12 months and serum follicle-stimulating hormone (FSH), luteinizing hormone (LH) and plasma oestradiol levels in the postmenopausal range for the institution for women under 50.

⁃ Ability to understand a written informed consent document, and able to comply with the protocol for the duration of the study including undergoing treatment and scheduled visits and examinations.