If they behave normally and they don't become lame, horses wont usually founder on oats. They will founder on corn and barely, but typically they will not founder on oats. Just keep an eye on them, and if any strange behavior appears, then get them checked out.
Horses absolutely can founder from oats. By the time you see symptoms it is too late in terms of lessoning the blow. Might as well get the vet our during the day before it turns into an emergency call.
Call the vet don't play the wait and see game once signs of founder show up its too late. If my horses get into the feed room and eat ten or more pounds of feed I have the vet ASAP no waiting to see what happens.
But in summer my horse got into oats and ate quite a few pounds and all that happened was that he was extreemly 'hot' and he flew around the pasture a couple of times. He never got sick but he didnt eat ten pounds....