Preheat oven to 225F. (107C) Thinly slice the washed sweet potato with either a sharp knife or a mandolin (with a guard).
In a medium sized bowl, add sweet potato slices, olive oil, salt, pepper and oregano and use a wooden spoon to mix. Mix well so that the sweet potato slices are evenly coated with olive oil.
Using 2 to 4 cookie sheets, lay the slices evenly on each cookie sheet — try not to overlap each slice. Bake in the 225F oven for 2 hours. Turn the oven down to 195F and bake for 30 minutes.