There are quite a few replies already but consider the basics. A horse whether fat or not needs the proper nutrients to remain healthy that often pyre pasture will not allow especially if it is reduced. An oats filler isn't needed when they are fat already and on pasture, you don't need the calories and the nutrients in 12 fluid oz of oats isn't a large benefit.
To reduce the weight while providing and adequate diet consider feeding a micro nutrient like KER IR Pellet
It was designed as a concentrated, low-calorie, low-starch source of vitamins and trace minerals. This stuff isn't a diet pill though
, to help reduce the weight add a grazing muzzle during the daytime and if needed take it off at night. Many horses especially IR horses and ponies on pasture with lower work loads need a grazing muzzle for much of the year. You may or may not need to go this route.