In a large bowl, mix wet ingredients - sourdough discard, melted coconut oil, cane sugar, brown sugar, and milk.
Gradually add dry ingredients - oats, flour, baking soda, and sea salt. Mix until just combined. Stir in raisins.
Refrigerate dough overnight for best results. If you are in a pinch, you can refrigerate the dough for a shorter period but don't go below 1 hour.
The next day, let the dough sit at room temperature for about an hour before you start turning it into cookies.