Ovarian cyst that may measure 30 cm or more in diameter, is most likely a benign ovarian cyst known as cystadenoma.
It develops from the ovarian tissue and may be filled with a mucous-type fluid material.
This could be also a dermoid cyst or endometrioid cyst, both benign in nature.
Large intra abdominal cyst may originates from other organs within the abdominal cavity, too, but, since you have posted on ovarian cancer forum, I assume it is a ovarian cyst.

Best wishes!
Marija