• Patient must have enrolled onto EAY191 and must have been given a treatment assignment to ComboMATCH to EAY191-N4 based on the presence of an actionable mutation as defined in EAY191

• Patients must be enrolled on the ComboMATCH Master Registration Trial EAY191

• Patients must have RAS pathway mutations as determined by the ComboMATCH screening assessment

‣ Cohort 1: Patients with histologically confirmed RAS pathway mutant ovarian, primary peritoneal, or fallopian tube (ovarian) cancer (activating mutations in KRAS, NRAS, HRAS, BRAF, MEK1, MEK2, or inactivating mutations in NF1)

⁃ Cohort 2: Patients with histologically confirmed RAS pathway mutant endometrial cancer (activating mutations in KRAS, NRAS, HRAS, BRAF, MEK1, MEK2, or inactivating mutations in NF1).

• Patients must have disease that can be safely biopsied and agree to a pre-treatment biopsy, if disease cannot be safely biopsied, or have archival tissue available from within 12 months prior to registration on the ComboMATCH Registration Trial (EAY191)

• Patients must have progressed after first-line treatment for recurrent or persistent disease

• Patients with ovarian cancer should not be eligible for further platinum-based therapy

• Patients with endometrial cancer must have received or been offered an immune oncology agent (alone or in combination with lenvatinib) unless there are existing contraindications for immune oncology agents or lenvatinib

• Patients may have received unlimited prior therapy

• Patients must have measurable and biopsiable disease. Measurable disease is defined by RECIST 1.1 as at least one lesion that can be accurately measured in at least one dimension (longest diameter to be recorded). Each lesion must be \> 10 mm when measured by CT, magnetic resonance imaging (MRI) or caliper measurement by clinical exam; or \> 20 mm when measured by chest x-ray. Lymph nodes must be \> 15 mm in short axis when measured by CT or MRI

‣ Patients must have at least one target lesion separate from the lesion to be biopsied to be used to assess response on this protocol as defined by RECIST version 1.1. Tumors within a previously irradiated field will be designated as non-target lesions unless progression is documented or a biopsy is obtained to confirm persistence at least 90 days following completion of radiation therapy

• Prior therapy must have been completed at least four weeks prior to registration

• Age \>= 18

• Eastern Cooperative Oncology Group (ECOG) Performance Status of 0, 1 or 2

• Hemoglobin (Hgb) \>= 9.5 g/dL with no blood transfusion in the past 28 days (within 14 days prior to registration)

• Platelets \>= 100,000/mcl (within 14 days prior to registration)

• Absolute neutrophil count (ANC) \>= 1,500/mcl (within 14 days prior to registration)

• Patients must have creatinine clearance estimated of \>= 50 mL/min using the Cockcroft-Gault equation or based on a 24 hour urine test (within 14 days prior to registration)

• Total bilirubin level =\< 1.5 x institutional upper limit of normal (ULN) or =\< 3 x ULN in the presence of documented Gilbert's syndrome (unconjugated hyperbilirubinemia) (within 14 days prior to registration)

• Aspartate aminotransferase (AST) and alanine aminotransferase (ALT) =\< 3 x ULN (within 14 days prior to registration)

• Patients must be able to swallow and retain oral medications and be without gastrointestinal illnesses that would preclude absorption of selumetinib or olaparib

• Patients with known history or current symptoms of cardiac disease, or history of treatment with cardiotoxic agents, should have a clinical risk assessment of cardiac function using the New York Heart Association Functional Classification. To be eligible for this trial, patients should be class 2B or better

• Women of childbearing potential (WOCBP) must agree to use two forms of birth control (hormonal or barrier method of birth control; abstinence) during the study and for 12 weeks after completing treatment

‣ Non-sterilized male partners of WOCBP (including males sterilized by a method other than bilateral orchidectomy e.g., vasectomy) who intend to be sexually active with a female partner must be using an acceptable method of contraception such as male condom plus spermicide (condom alone in countries where spermicides are not approved) from the time of screening throughout the total duration of the study and the drug washout period (at least 16 weeks after the last dose of study intervention) to prevent pregnancy in a partner. Periodic abstinence, the rhythm method, and the withdrawal method are not acceptable methods of contraception. Vasectomized (i.e., sterile) males are considered fertile and should still use a male condom plus spermicide as indicated above during the clinical study

• Patients with a prior or concurrent malignancy whose natural history or treatment does not have the potential to interfere with the safety or efficacy assessment of the investigational regimen are eligible for this trial

• Human immunodeficiency virus (HIV)-infected patients on effective anti-retroviral therapy with undetectable viral load within 6 months of registration are eligible for this trial

• Patients with evidence of chronic hepatitis B virus (HBV) infection must have an undetectable HBV viral load on suppressive therapy, if indicated

‣ Patients with a history of hepatitis C virus (HCV) infection must have been treated and cured. For patients with HCV infection who are currently on treatment, they are eligible if they have an undetectable HCV viral load

• Patients with new or progressive brain metastases (active brain metastases) or leptomeningeal disease are eligible if the treating physician determines that immediate central nervous system (CNS) specific treatment is not required and is unlikely to be required during the first cycle of therapy

‣ Patients with treated brain metastases are eligible if follow-up brain imaging after CNS-directed therapy shows no evidence of progression

⁃ Extra caution should be taken with olaparib, as it crosses the blood brain barrier and can cause edema in brain metastases

• The patient or a legally authorized representative must provide study-specific informed consent prior to study entry and, for patients treated in the United States (U.S.), authorization permitting release of personal health information