I may be wrong here as, despite living in Australia my interest leans more towards the British & Indian made Lee-Enfields, but my understanding is that MA is "Manufacturer, Australia" (as you note, Lithgow) with OA being "Orange, Australia". Ian Skennerton's book makes reference to MAO for Orange as well.
There should be a series of stars with letters inside them stamped along parts of the action as well - these are inspection marks from various stages of the manufacturing process.