Totally understand your concern Helen as far as the expanding in the belly thing.
I may be way off on this but using logic.... we're not supposed to throw rice at wedding as it expands in the gut of birds once eaten. Makes sense as it's raw, hits the stomach fluid, absorbs, expands....
I would think that with rice or oats, anything that expands when mixed with water, once mixed with said water and the expansion has taken place, it should be alright.
I did mix mine up some oatmeal with strawberries the other day. Hubby had made some and I snatched a scoop out. I let it soak and the gliders didn't eat it right away so it was as "absorbed" as it was gonna get. They liked it. Ate almost the entire large scoop I gave them.
Now I can't say whether or not it's good or bad for them but they had no bowel disruption or anything after eating it. Of course it isn't something I would offer a lot but as a now and then thing I think it's alright.
As with us, all things in moderation.