To help with constipation, you can add an extra ounce of water to your lo's bottle. I think it takes a little while for babies tummies to get use to solids because this is the first time they have had to digest anything other than milk. Plus, since you just recently started formula that can cause some constipation. Hopefully, by adding some extra water, your lo won't have such a hard time. I don't know if oatmeal is less constipating than barley. My son has had less of an issue with constipation since we have added in fruits and veggies. I know pears, peaches, plums, and prunes are good for constipation, once your lo is ready to start fruits and veggies. Squash also seems to really help get my son's system moving. As for the little amount she is eating.... just hang in there and keep trying and she will increase the amount as she is ready. It is a process. Good luck to you!