Place rack in center of oven and preheat to 375°F. Line a large baking sheet with parchment paper or a silpat mat.

In the bowl of a standing mixer fitted with the paddle attachment or a large mixing bowl, beat together the oil and peanut butter on medium speed until smooth and combined. Beat in the granulated sugar and brown sugar until well combined. Add the vanilla and egg, beating well to fully incorporate.

In a separate medium bowl, whisk together the whole wheat pastry flour, flaxseed, baking soda, and salt. Add the flour mixture to sugar mixture, beating at low speed just until combined (mixture will be very thick and may be crumbly). Stir in chopped peanuts.

Scoop the dough by rounded tablespoons lightly packing it as needed so that the cookies hold together. Drop cookies 1 inch apart on the prepared baking sheet. With a fork, gently press the top of each cookie one direction, then the other to form a crisscross pattern.

Bake for 8 minutes or until lightly golden and the tops look slightly dry (cookies will appear underdone). Let cookies cool 2 minutes on the baking sheet, then remove to a wire rack to cool completely.