O

Oestrogen/Oestradiol: Female sex hormone produced by the ovary.  Levels fluctuate during the menstrual cycle.

Oligozoospermia: Low sperm count - less than twenty million sperm per millilitre. It can be severe - less than five million sperm per millilitre. 

Oocyte: The female gamete (egg).

Ovaries: The female reproductive organ in which oocytes are produced from pre-existing germ cells. The ovaries also produce hormones.

Ovarian Hyperstimulation Syndrome (OHSS): A serious complication following stimulation of the ovaries with gonadotrophin drugs. 

Ovulation: The release of an egg from a follicle in the ovary. 

Ovum: The female gamete (egg).