Can I bake oatmeal drop cookies in a bar form?
Most drop cookies that are made by creaming butter and sugar turn out fairly well as bar cookies. 

One thing to think about is the additional baking time - since they are thicker, you may have an underdone center even once the top is golden brown. 

You should probably bake them slightly longer than you would normally, and check them fairly frequently. I prefer my cookies chewy (almost like cookie dough) in the middle, so I tend to bake them for less time.