The short answer is that anyone who is quite familiar with Excel would
easily recognize the basic structure of the Open Office spreadsheet. They
might not like it as well at first as there are differences, the main one
being incompatible macros - so I'm told, I've never used them myself.

They should however be able to adapt easily, if they are prepared to give it
a fair try. Lets put it this way: OO Spreadsheet is more like Excel than (K)
Ubuntu is Windows.