What is the difference between oats and oatmeal?

Oats are the grain; oatmeal usually means oats prepared as a cooked cereal and can also refer to a processed oat product.

Whole oat grain and a prepared bowl of oatmeal — Oats name the grain; oatmeal commonly names the prepared cereal.

The short answer

The word oats can describe oat groats and the foods made from them. Oatmeal usually describes a cooked porridge, although labels may use the word for rolled or ground oat products. Check the ingredient list and Nutrition Facts panel because preparation, added ingredients and serving sizes differ.

Sources

Food Labeling GuideU.S. Food and Drug Administration