Position rack in center of oven and preheat to 350°F. Generously butter a rimmed 12x18x1-inch baking sheet; set aside.
In a medium bowl, whisk together flour, baking soda, salt, cinnamon, ginger, nutmeg, and allspice. Mix in oats until well-blended. In a large bowl, use an electric mixer to stir butter until creamy. Blend in sugar and brown sugar and beat on medium-high speed for 3 minutes or until light and fluffy. Mix in the eggs, one at a time, until combined, then add the vanilla and pumpkin and mix until smooth.
Blend in the flour mixture until just combined, and stir in the chocolate chips. Spread in an even layer in the prepared pan.
Bake for 25 to 30 minutes or until light golden brown and a toothpick inserted in the center comes out just clean (do not overbake). Cool in the pan on a wire rack. Cut into 24 or 36 bars, depending on what size you like them.