Your oat bread is a quick bread recipe. Keep in mind that advice for quick breads is going to be very different than advice for yeast risen / glutinous breads.
In quick breads, the structure of the loaf is provided by egg protein and the moisture is mostly provided by fat.
Your recipe uses only egg whites which, as J K commented, have a drying effect on baked goods. The yolks in whole eggs have fat that help keep things moist.
Besides the water in your whites there is no other liquid and no fat in this recipe. I'm surprised that you were able to swallow it at all. This is a problem that plagues low calorie baked goods. You can make it a bit better than this recipe but don't expect it to ever be as good as recipes that have fat in them.
This recipe is so dry that I would have a hard time making suggestions on how to improve it without turning it into a new recipe that would have to be tested.
Some things to look for in recipes that will make them better:
Some added fat
It doesn't have to be a lot. Some quick breads have a lot of fat. Even a little bit will help. Some recipes will use part fat and part applesauce. The pectin in the applesauce helps make things tender and the extra liquid helps keep things moist.
As I said above, whole eggs are also nice for some good fats.
Less of the unswallowable ingredients
Those psyllium husks are going to be bone dry no matter what you do. If there is a lot of that in there have a glass of milk handy when you try and swallow it.
Not all recipes use added liquid but some like buttermilk can be very tasty additions.
My personal opinion is that I would rather eat less of something that tastes good than a lot of something gross. Low-calorie can be taken too far.
Another alternative is to abandon the bread idea altogether. This recipe would be very similar to low-calorie granola recipes if rolled out thinly.