Preheat oven to 375°. Spread 2 cups (178 g) old-fashioned oats on a rimmed baking sheet and toast until lightly golden, about 6 minutes. Add 1 cup (100 g) pecans, chopped, toss to combine, and continue toasting until pecans are fragrant and oats are deep golden brown, about 6 minutes more. Let cool sightly.

Whisk 1 cup (125 g) all-purpose flour, 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt, ½ tsp. baking soda, ½ tsp. ground cinnamon, and ⅛ tsp. freshly ground nutmeg in a medium bowl; stir in cooled oats and pecans.

Using an electric mixer on medium speed, beat ¾ cup (1½ sticks) unsalted butter, room temperature, ¾ cup (packed; 156 g) light brown sugar, and ¼ cups (50 g) granulated sugar in a large bowl, scraping down sides of bowl, until light and fluffy, 3–4 minutes. Add 1 large egg, room temperature, 2 Tbsp. pure maple syrup, and 2 tsp. vanilla extract or paste and beat until incorporated, about 1 minute. Reduce speed to low; add flour mixture and 1 cup raisins, dried sour cherries, and/or dried cranberries and continue to beat, scraping down sides and bottom of bowl, until just combined. Let sit at room temperature at least 1 hour to hydrate oatmeal.

Preheat oven to 350°. Line 2 rimmed baking sheets with parchment paper. Using a 2-oz. ice cream scoop (or ¼-cup measuring cup), portion cookies and place on prepared baking sheets, spacing 3" apart (cookies will spread as they bake).

Bake cookies, rotating cookie sheets halfway through, until edges are golden brown and firm but centers are soft, 15–17 minutes. Let sit on sheets 10 minutes, then transfer to wire racks; let cool.

Do ahead: Cookie dough can be made 3 days ahead; store chilled in an airtight container. Let dough come to room temperature before baking.