Can I use Natural Cycles if I have 1 ovary?
Yes, you can use Natural Cycles even if you only have one ovary (such as if you've had an oophorectomy). This is as the remaining ovary will still keep producing eggs, and will usually start ovulating every cycle instead of every other cycle to compensate for the missing ovary. For women who are born with only one ovary, she will most likely ovulate normally from the first period and onwards.
Becoming pregnant is still possible with one ovary, and is usually no more difficult than with two ovaries. This is dependent on a healthy and intact fallopian tube. Without the fallopian tube, a natural conception is very unlikely.
Ovary removal is mainly used to treat or prevent problems regarding the woman's reproductive organs (uterus, fallopian tube, cervix, ovaries). Such problems is ovarian cancer, ovarian- and breast cancer prevention, or in the event of ovarian torsion (twisting of an ovary).
How will this work with Natural Cycles?
You will usually still ovulate, and thus Natural Cycles will be able to detect your ovulation. The remaining ovary will still produce estrogen and progesterone meaning that your temperature will still follow a curve, and increase after ovulation has occurred. The detection of your ovulation will allow the app to give green days and the data is used to give you a good prediction for the coming cycle too.
You can therefore still Prevent or Plan a pregnancy with Natural Cycles.