Offensive/defensive is mostly for determining whether or not there will be a battle. But since you're running a TO, you're obviously at war, so there *will* be a battle, regardless of offensive/defensive settings.
At that point, the encounter reaction only affects how fast your men will leave fortifications, provided they have any. If you don't have the walls (which you won't), and you're not dug in, then offensive/defensive settings have no effect on the battle at all.