X means cross as in crossing a boundary, crossing over or "trans" as transdimension or transformation - X-men are transformed - Professor X (Xavier) - (“An X is an ancient symbol for change or transformation.. .long associated in medieval and renaissance art with the coming of the Messiah who shall make all things new.” —Jim Tresner, 33° Scottish Rite Journal)
With an O, the X is a symbol associated with the mark of the beast.