I would up the oatmeal so it is 1 more than the flour and bring the brown sugar to 1 so it is equal with the flour. So it would look something like this...
2.5 cups oatmeal
1.5 cups flour
1.5 cups brown sugar (or you could try 1 cup brown sugar and half a cup of granulated sugar)
The extra brown sugar will help make the cookie softer and more chewy. You could also add a packet of instant jell-o pudding mix without having to change the recipe you are currently using.
I would also pay attention to how you are storing them. To get the most life out of them keep them in an air tight container with a slice of bread.