Pre-heat oven to 350 degrees. Line two large cookie sheets with parchment paper or waxed paper.
Either by hand or with electric mixer, beat butter until creamy.
Add sugars; beat until fluffy, about 3 minutes.
Beat in eggs one at a time.
Mix flour, salt, baking powder, and nutmeg together in medium bowl.
Stir dry ingredients into butter-sugar mixture.
Stir in oats, raisins and optional walnuts.
Working with generous 2 tablespoons of dough each time, roll dough into 2-inch balls. Place balls on parchment-lined cookie sheet, leaving at least 2 inches between each ball.
Bake until cookie edges turn golden brown, 22 to 25 minutes.
Let cool on cooling rack at least 30 minutes before peeling cookie from parchment.
If you prefer a less sweet cookie, you can reduce the white sugar by one-quarter cup, but you will lose some crispness. Do not over bake these cookies. The edges should be brown, but the rest of the cookie should be very light in color. The trick to making the cookies chewy is to make them large. Smaller cookies tend to get more dried out and crisp, and therefore not as chewy.