Es carnero y cordero la misma cosa'. No estoy segura
Normally in Spain you understand "cordero" as a lamb or a mutton. But when you think about a "carnero" you think about a ram, a mature animal with snail horn.
Yo can try a visual example entering "cordero" and "carnero" on the spanish google image search.
This is probably a regional thing
I've heard that they mean the same thing, but
carnero is a ram and
cordero is a young lamb
So if you are talking about food, I would think of carnero as mutton and cordero as spring lamb. In one dictionary I consulted it explained cordero as lamb & mutton, so I guess it's your choice.
oveja is sheep but I think that means it's on the hoof & not your dinner.
It will be interesting to hear what Spain & L..Am.have to say.