Hmm well we're taught this way in Francei take it that Oy is the y axis (god knows why)

"y" goes up and "x" goes left. O is the origin.

Ox denotes the absciss axis and Oy denotes the ordinate axis.

A reflection by Ox gives -f(x)

Yes, if g denotes the function of the curve reflected in Oy, we have g(x)=f(-x)