Upon your second visit to OsTown Goofy will have a quest for you called 'Goofy's Fountain Mishap'. You'll need to go into the sinkhole in the middle of the map and recover the statue pieces by moving them to Transport Tubes. At the very bottom of the sinkhole you will find two paths, one leads to a Mickey head and the other to Oswald.
Collect the heads and then go and put Oswald's head on the town statue. The third time you come back to Ostown you will find the Gag Factory where Gremlin Maroon can build a Mickey statue in exchange for scrap metal. This will give you the Heroes of Wasteland pin and achievement.