Why are oxides always of the form AyOx with A representing any atom except oxygen and fluorine
Generally the least electronegative element is written first. And while naming also, +ide is attached as prefix to the more electronegative element.
For Ex- $ICl$ is written as Iodine Chloride not Chlorine Iodide. Similarly $OF_2$ is written as Oxygen Difluoride.
Coming back to your question, Oxygen is the second most electronegative element in the periodic table after Fluorine. Therefore it is generally written after all the other elements (except fluorine).