Oats are safer to feed than most commercial feed products. They are a pure "whole food" that is healthy for the horse. If you're feeding quality Timothy hay, oats are actually the perfect thing to "balance" the diet. The only thing you'd be low in is Vitamin E and Calcium, and not by a lot. Adding some Alfalfa remedies that.
Horses are in good condition when their coats have a glow, and the hooves are strong, and the eyes are bright. A good diet is the first step to a healthy horse.