Stir the flaxseed with the water in a small bowl and set aside to gel for 5 minutes. Meanwhile, preheat the oven to 350°F (175°C) and line a baking sheet with parchment.

Whisk the melted coconut oil and brown sugar in a large bowl until smooth, then whisk in the flax egg and vanilla.

Add the flour, baking soda, cinnamon, and salt to the wet mixture and stir until just combined. Fold in the oats and any optional raisins or chips.

Drop rounded tablespoon-sized scoops of dough onto the baking sheet, spaced 2 inches apart. Bake for 10 to 12 minutes, rotating halfway, until edges are golden and centers are soft. Cool on the sheet for 5 minutes before moving to a rack.