Good luck!!!!Hope all goes well for you!
If it is a general lap to look for endometriosis or something, no, you don't have to agree in advance to the possible removal of an ovary. If it is a specific lap because they suspect something is wrong with the ovary and it may be important to take it out, then doing it in one step rather than waking you up and making you wait is smart. (Sometimes this pre-approval happens when doing exploratory surgery for suspected breast cancer, but they only do that as one step if they are pretty darn sure going in that the breast has to be removed.) Good luck!!!
Best of luck with your surgery and I hope you have a speedy recover! I'm not sure about the ovary removal thing. They might just make you sign that because if they open you up and see that it is more of a problem than they thought and that it will need to be removed they'd probably (legally) need your consent prior to be under the anaesthetic and at the time of the surgery. It might not have to be removed though.