$\underset{O}{O}\frac{x}{y}$ ${\displaystyle \underset{O}{O}\frac{x}{y}}$ $\begin{array}{c}\underset{O}{O}\frac{x}{y}\end{array}$ $O_O\frac{x}{y}$ $O_O\frac{x}{y}$ $\begin{array}{c}{O}_O\frac{x}{y}\end{array}$