My grandmother used shortening, not butter, when making these cookies. These days I almost always use butter. Either will do; the shortening cookies I think tend to be a bit chewier. By the way, if you make the cookies extra large, they will be chewier, just cook them longer (20 min instead of 10).
Do not overbake these cookies. The edges should be brown, but the rest of the cookie should be very light in color.

INGREDIENTS
1 cup (1/2 pound or 2 sticks) unsalted butter, softened, OR 1 cup shortening
1 cup brown sugar, packed
3/4 cup granulated sugar
1 1/2 cups all-purpose flour
1 teaspoon salt
1 teaspoon baking soda
1/2 teaspoon freshly grated nutmeg
1 teaspoon cinnamon
2 large eggs, well beaten
1 Tbsp vanilla
3 cups rolled oats (We use Quaker Quick or Old Fashioned. Do NOT use instant.)
1 1/2 cups raisins
1/2 cup chopped walnuts (optional)
METHOD
1 Pre-heat oven to 350 degrees. Grease two large cookie sheets or line with Silpat, parchment paper or waxed paper.
2 In a large mixing bowl, beat butter until creamy. Add sugars; beat until fluffy, about 3 minutes. Beat in eggs. Add vanilla.
3 Mix flour, salt, baking soda, cinnamon, and nutmeg together in medium bowl. Stir dry ingredients into butter-sugar mixture. Stir in raisins and nuts. Stir in oats.
4 Spoon out dough by large tablespoonfuls onto prepared cookie sheets, leaving at least 2 inches between each cookie.
5 Bake until cookie edges turn golden brown, 10-12 minutes. Cool 1 minute on cookie sheets. Then carefully remove them, using a metal spatula, to a wire rack. Cool completely. They will be quite soft until completely cooled. Store tightly covered.
Makes about 2 dozen cookies.
References :
http://simplyrecipes.com/recipes/oatmeal_raisin_cookies/