Hi Lorraine;


  If you mean Landau's BIg Oh notation then this is straight definition stuff. Best way to deal with this type of problem is research.
go here:
http://en.wikipedia.org/wiki/Big_O_notation
You will now be able to draw a graph of some f(n) and its O(n)

When you understand that and if this is for computer science then go here for a discussion on its weaknesses:
http://leepoint.net/notes-java/algorithms/big-oh/bigoh.html

If you get past this you are ready for the Physicists. They are having an interesting discussion on Big Oh notation and computation of factorials. Careful though, some of it is not correct.
http://www.physicsforums.com/showthread.php?t=280854

Then comeback and you can explain it to me.