Additional Study Information: The purpose of this study is to compare any good and bad effects of using olaparib and atezolizumab in combination compared to olaparib alone. The use of olaparib alone or in combination with atezolizumab could shrink your cancer but could also cause side effects. This study will allow the researchers to know whether the combination approach is better, the same, or worse than olaparib alone. To be considered better, the study drug combination should increase the amount of time that a persons cancer does not grow by at least 5 months compared to the single drug approach. Olaparib has been approved by the United States Food and Drug Administration (FDA) for treatment of certain kinds of ovarian and breast cancers. Atezolizumab has been approved by the FDA for treatment of urothelial and certain kinds of lung cancer, but is not approved by the FDA for treatment of breast cancer. There will be about 72 people taking part in this study.