大丈夫とか言ってる前に、ドキュメント嫁よ。

C-x o => other-window

other-window is an interactive built-in function.
(other-window ARG &optional ALL-FRAMES)

Select the ARG'th different window on this frame.
All windows on current frame are arranged in a cyclic order.
This command selects the window ARG steps away in that order.
A negative ARG moves in the opposite order. If the optional second
argument ALL_FRAMES is non-nil, cycle through all frames.

要は、「A negative ARG moves in the opposite order.」だから、

C-u - C-x o

だろ。