The operation of a horizontal rolling (horizontal sliding) window employs an fixed sash on one side and an operable sash on the other side which rolls in front of the inoperable sash. What this means is that, when looking at the window, one side will open and the other will not.
The way glaziers talk about this difference is to call the stationary sash "O" and the operable sash "X". No, the operable sash is not denoted with an "O", because glaziers want to be difficult.
Anyway, when viewed from the outside, an XO horizontal sliding window will have the left side openable, while the right side remains fixed.
With windows, left and right is always defined by the point of view of the glazier, namely, from outside. So, an OX window configuration will have the right pane operable, and the left sash inoperable when viewed from the outside.