Preheat oven to 225 degrees.
Slice the tomatoes into ½ inch thick slices and toss with the oregano, basil, garlic powder, salt and pepper. Place a wire rack on top of a baking sheet. Place the tomato slices on top of the wire rack and bake in the preheated oven. Check the tomatoes after 3 ½ hours. The tomatoes will dry out a bit faster in a convection oven. If the tomatoes are not dried, put back in the oven for another ½ hour.
Remove and let cool. Can be made a day ahead. If making ahead, store in a covered container at room temperature.