Was told i have a 8.4cm and 4cm possible endometriomas and endometriosis. obgyn wants to do lap cystectomy while gyn/onco wants to do laparotomy. why?

Hard to say: Typically, a less invasive surgery is better. You should request a second opinion from another GYN oncologist. It is impossible to render a good second opinion without knowing much more about your history, your physical exam and knowing the lab / radiology results. Some older physicians who do not perform as many laparoscopic surgeries will be more likely to recommend laparotomy. Best of luck!