Flour, oats and barley
If one of the five grains – wheat, barley, rye, oats and spelt – sits in water for more than 18 minutes it becomes chametz, and one may not eat, derive benefit from or own it on Pesach. In addition, Ashkenazim don’t eat kitnios – a group of foods which includes (among other things) rice, corn, soy and their derivatives – but are allowed to own kitnios foods on Pesach.
It is common practice that before wheat is ground into flour, the wheat kernels are tempered with water for many hours, and therefore flour should be treated as chametz (Mishnah Berurah 453:24). [In fact, matzah which isn’t baked especially for Pesach is made from tempered wheat and should also be treated as chametz!]. Similarly, all oats are heat-treated to prevent them from becoming rancid; if this heating is done with “wet” steam, the oats/oatmeal may be chametz. On the other hand, barley (a.k.a. pearled barley) is processed without water, and therefore a standard bag of barley isn’t chametz. However, some barley is steeped in water until the barley begins to sprout; this creates a product known as barley malt (a.k.a. malted barley, malt) which is definitely chametz.
So, sometimes yes, sometimes no. You have to check the box for "Kosher for Passover" or "kasher l'pesach"