I think .map is probably fine as it is. Like said before it describes the file's contents well enough.
Even if we were to change it, we could only do it on maps created from now on otherwise we create big compatibility issues (and more complaints. Sort of like changing on6_01.map -> opu_05.map or whatever, but the consequences would be even greater).
No problem with doing it on new maps. But then you have a uniformity issue (if you're a purist
). Also some tools might need to be changed to have *.o2m or whatever files listed in open and save dialogs (otherwise the user has to manually input the filename, or set it to All Files)