Oven-ready food has already been prepared when you buy it, so you only have to put it in the oven and cook it. A common example is oven-ready chicken: the shop has already cleaned it and often taken out the insides, so you are not doing that messy work at home. Frozen lasagne, some pies, and tray-bake vegetables are sold this way too. The word is especially at home in British supermarket language. It does not mean the food is already cooked — that would be ready-to-eat — and it does not mean your oven is hot. You still have to cook it; the “ready” is about the prep, not the eating. You will see it on packets more than you will hear it in a story.