When did you get your ileostomy? Is it a new ostomy?
In general, they advise low residue GI diet for about 6 weeks after a new ostomy and oatmeal would be a no. But after that initial 6 weeks, assuming all is well with output etc, then you can start trying 1 food at a time. They had my daughter add only one food a day and small amounts of it to see if it caused any problems. Then only add that food for about 3 days. If it goes well, safe to proceed to the next. If you are craving a hot cereal, Cream of Wheat is o.k. the first 6 weeks.
In general, oatmeal is pretty safe and probably a good first food to try. It comes in super handy for thickening loose output and is pretty well digested and passes easily.
Foods to wait a good long while on and maybe make the last adds are lettuce, raw veggies, raw fruit etc…some people are never able to add these as they are troublemakers but my daughter does o.k. with small amounts of them. She still hasn't been able to add strawberries and it has been a year. Also her surgeon gave her a hard NEVER for nuts and seeds.
Chewing your food really well is key with an ostomy. Chew, chew, chew and then when you think you are done chew some more.
My daughter had a loop ileostomy for the past year and just converted it to a permanent end ileostomy. The dietician in the hospital hasher starting over but her surgeon felt after 2-3 weeks that she could go back to her regular diet as the ostomy wasn't brand new and we just needed a few weeks to make sure the new one was working.