Preheat the oven to 375 degrees Fahrenheit. Line a rimmed half cookie sheet (13x18x1) with parchment paper, or spray with nonstick spray.
In a large bowl, combine the old fashioned oats, flour, baking powder, and salt. Set aside.
In a large mixing bowl, mix together your peanut butter and butter.
Add in the sugar and brown sugar and cream together till smooth, and light.
Add in the eggs, and vanilla and stir to combine.
Add the dry ingredients to the wet ingredients and stir it all together.
Press the cookie dough into the cookie sheet, spreading evenly to the edges. The dough is pretty sticky, so spray your hands or a rubber spatula with nonstick spray while you spread it.
Bake in the preheated oven for 15 minutes, until the middle is just barely set.