Oden could have :
-> destroyed the floor, making the barrier guy fall and defeated Orochi
-> Calling a retainer, or any soldier really to fight barrier guy while he defeat Orochi
-> Used Conqueror Haki, and Defeat Orochi
-> Make the ceiling falls on them, and defeat Orochi
-> Get out of the building and smoking the castle, an when the rat get out of his hole, defeat Orochi