The people at Levels, who do continuous glucose monitoring, say that oatmeal is one of the worst offenders.

You really need to spike your oatmeal with fats, protein, and fiber.

I regularly eat steel-cut oats for breakfast. My go-to is steel-cut oats, two tablespoons of Chia seeds mixed in (tons of fat and fiber), and a serving of mixed nuts and berries on top. On the side, I eat four scoops of powdered peanut butter mixed with water. This version of peanut butter is much higher in protein and much lower in fat and calories than normal peanut butter.

Doing all of this can keep the spikes to a reasonable amount (I also eat the powdered peanut butter first). Any kind of oatmeal by itself is bad. Instant oatmeal is worse. Instant oatmeal with all that sugar and stuff thrown in is terrible.

I suspect if you made your oatmeal with milk instead of water, it would help a lot, but I can't do this from being lactose intolerant.