They were named X and Y chromosomes Because the time of discovery they didn't know what they did.So they chose XandY because in math your always solving for x or y.
XO is a called Turner syndrome. Heres a bit from an article I thought was interesting:
Sex chromosome abnormalities are more common and tend to have less-drastic effects than autosomal abnormalities. Normal females have two X chromosomes, and males have an X and a Y; abnormalities in sex chromosome distribution produce Turner’s syndrome (XO), Klinefelter’s syndrome (XXY), and the so-called supermale (XYY). Turner’s and Klinefelter’s individuals have female and male genitalia, respectively, with retarded development of sexual characteristics. Supermales tend to be taller than average and to have learning disabilities. Although some studies have suggested that there exists an association between supermaleness and criminal behaviour, the link has been largely rejected. In fact, many XYY individuals are socially well adjusted.