Preheat the oven to 350°F (180°C, gas oven brand 4). Lightly oil three cookie sheets or line them with parchment paper.
Whisk the ground flaxseed and water in a mini blender, the small bowl of a food processor, or with a whisk until thick, creamy, and somewhat gelatinous. Beat the butter, the two types of sugar, the vanilla, and the flax seed mixture (the flax egg) by hand or using an electric hand mixer in a large mixing bowl until the mixture is homogeneous.
Combine flour, oat bran, baking soda, baking powder, salt, cinnamon, and nutmeg well in a separate medium bowl. Add to butter mixture and combine well until combined and smooth. Stir in rolled oats and chocolate chips until well blended.
Scoop out some dough using a tablespoon or ice cream scoop and, with lightly oiled hands, gently press the cookie dough into rounds about 1 cm (½ inch) thick and spread on the cookie sheet. Bake until cookies are golden brown, 12 to 15 minutes. Remove from the oven and let the cookies firm up for a few minutes on the cookie sheet. Transfer the cookies to a cooling rack. Sprinkle with flaky sea salt, if desired.
Stored in an airtight container, these Vegan oatmeal chocolate Chip cookies will keep for up to 5 days at room temperature, in the fridge for up to a week, or in the freezer for up to three months.