Always buy the mercury's treads and the frozen mallet. Once you have that completed you can kind of mess with your build. If you are the "Tank" of your team the I suggest buying Rainduin's Oden followed by a warmogs armor with the maw of malortius and complete your build with either another frozen mallet or a trinity force. But I recommend that you always buy the mercury's treads and frozen mallet every time you play as Olaf.