Why is oatmeal kosher for pesach?

Hello, I was looking at the kosher-for-pesach list for this year, and I was wondering, why is oatmeal considered kosher for pesach? Isn’t it considered one of the five grains that are considered chametz on pesach?

Heating up matzah in the oven

Is it assur to heat up matzah in the oven? During Chol HaMoed Let’s say I want to make a pizza with Matzah and I heat up square commercial matzah for 10-15 minutes. Would that be considered an extension of the the baking time past 18 minutes, or is it ok because...