The purpose of this study is to learn more about the investigational study drug called OP- 1250 (palazestrant) on participants with ER+, HER2- advanced or metastatic breast cancer. OP-1250 is considered endocrine therapy (treatment that adds, blocks, or removes hormones). Estrogen is a type of hormone naturally produced by the body that helps develop and maintain female sex characteristics and the growth of bones. Estrogen can also cause certain types of breast cancer to multiply, grow, and move to different parts of the body. Estrogen works in the body by binding to proteins called estrogen...