They sit side by side on the shelf and often share ingredients, but oats, muesli and granola are made differently, and that changes how you use them.
Oats
Oats are the grain itself, rolled or cut. On their own they are a blank canvas: warm them with milk or water, soak them, or use them in baking.
Muesli
Muesli is a mix of raw or lightly toasted oats with other ingredients such as nuts, seeds and dried fruit. It is not baked together, so each ingredient keeps its own character. It can be eaten cold with milk or yoghurt, soaked, or warmed.
Granola
Granola is baked. The oats and other ingredients are usually mixed with a binder, often a sweetener and oil, then baked until crisp and clustered. It is typically eaten cold, as a cereal or a topping.
Where oat blends fit
Blends of oats with nuts, seeds and fruit sit close to muesli: unbaked, with each ingredient doing its own job. The difference lies in how the combination is chosen and balanced.
Whatever you choose, the ingredient list tells you what you are actually eating. It is always worth a read.