Before they're fit for human consumption, whole oats must be cleaned, toasted, hulled, and cleaned a second time. At this point, they are called oat groats.
Rolled oats, also known as old-fashioned oats, are oat groats that have been steamed and flattened into flakes with large rollers. They take about 15 minutes to cook into oatmeal, but they can also be added raw to recipes.
Quick-cooking rolled oats are similar to old-fashioned oats, but they’ve been cut into pieces before being steamed and rolled, so they’re thinner and cook in about 5 minutes. Though they cook in about 5 minutes (versus the usual 15 for regular rolled oats), quick-cooking rolled oats lack some of that satisfying oatmeal flavor and texture. Use them as you would old-fashioned oats.
Steel-cut oats, also known as Scottish oats, pinhead oats, and Irish steel-cut oatmeal, are made from whole oat groats (cleaned, toasted, and hulled oat grains) that are cut with steel blades. Because they are cut—not rolled flat into flakes—they retain a chewy texture. They must be cooked before eating, but you can soak them in water first to reduce their 30-minute cooking time. There are also quick-cooking and even instant versions available.
Instant oats are cooked and dried before being rolled, which softens them enough that they need only about 90 seconds of cooking. They’re often packaged with additional salt, sugar, and flavoring, though, so look for plain versions to keep their health quotient high. Don’t use them as substitutes for old-fashioned or quick-cooking oats, because they tend to absorb more moisture and can alter the texture of your recipe.
Rolled oats and quick-cooking rolled oats can often be substituted for one another in recipes. Instant oats can not be substituted for either. Instant oats are oat groats that have been cooked and dried before being rolled. Precooking the oats softens them, so when they are mixed with wet ingredients, say in a baking recipe, they turn your baked goods into mushy lumps.