Preheat oven to 350 degrees. Line two baking sheets with parchment paper.
In the bowl of a stand mixer or with a hand mixer, cream coconut oil and brown sugar until fluffy, about 2 minutes. It may not get as fluffy as with butter, but a creamier texture should arise with enough whipping. Beat in oat milk, mint extract, and vanilla until smooth.
In a separate medium bowl, whisk together flour, baking soda, and salt. Pour dry ingredients into wet and beat until just combined. Some small streaks of flour are okay.
Fold in pistachios and dark chocolate chips until evenly distributed.
At this point, you have the option to chill the dough for 20-30 minutes if you want the cookies to spread less. I like spread, so I proceeded with baking immediately. When you're ready to bake, roll dough into heaping 1 tablespoon-sized balls and place onto baking sheet with about 2 inches of space between them.
Bake cookies for 7 minutes, then remove from the oven and tap a few times against the counter to flatten. Bake an additional 2 minutes. Cool on the baking sheet for 5 minutes, then slide the cookies to a wire rack to set at least 10 more minutes.