So I have this preference for oatmeal cookies, but want to increase the fiber content. How to sub oat bran for some of the oat meal?

If you didn't also post this in Food & Drink>Cooking & Recipes, do so. Those folks may be more helpful. I did find this reference in the source article "1 cup of oat bran can be replaced with 1 or ¾ cup rolled oats." so I'm assuming the reverse is true and you can replace 3/4 or 1 cup of rolled oats with 1 cup of oat bran (or basically a 1:1 ratio or a 0.75:1 ratio.

Instead of oat bran, consider oat fiber (even more insoluble fiber than bran) as a substitute for sugar.

Just halve the volume of the sugar of oat fiber and add a few drops of stevia extract.

Your cookies will be more moist and definitely not killing you.