Oatmeal would usually be fine after 16:8. It may just be that your stomach is still getting used to the new schedule, and the high-fiber oatmeal just freaked out your stomach a bit.
Maybe your body just needs time to adjust to IF. That’s super common when people start IF. Your GI tract is used to a certain schedule, and takes a little time to adjust to the new schedule — usually a week or two.
Honestly when in adjustment phase any meal upset my stomach until my body got use to the change.
If other foods don’t cause the same reaction, you could eliminate that meal and slowly reintroduce each item to find the culprit.
It might be the almond milk try make it out of water. Truth is almond milk isn’t that good for you . Contains very little almond. , mostly just b.s. Ingredients. Also the type of oats make a difference you want steel cut and not instant oats. You want no added sugars or flavor. If you want some sweet add brown sugar or fruits.