If there is, we do not know of one yet. But we must emphasize the YET. The technology to determine the composition of the atmosphere of an exoplanet is just now becoming sufficiently good enough to detect free oxygen, diatomic oxygen O₂.

In our solar system NO OTHER planet has free diatomic oxygen in its atmosphere. We believe that free oxygen originates exclusively by photosynthesis. Therefore if we did detect free oxygen in the atmosphere of another planet, we would have strong reasons to surmise, but not prove, that life existed there.

In science, we do not have all the answers. But the FUN part is that we can ask questions and then use our powers of reasoning and the experimental method to address very difficult questions. In many cases, if we study long and hard enough we can answer these questions.

None that we know of - there probably are planets in the universe with atmospheres that we can breathe, but we have not yet found them. Our atmosphere can only be breathed because of life on Earth (namely, plants and algae producing oxygen). So far Earth is the only planet we have discovered that has life on it.

There might be one out there somewhere in the universe, but none of the other planets in our solar system have an atmosphere that we can breathe. We need both enough oxygen so that we are not asphyxiated (20% oxygen or more) and there needs to be no toxic gases like sulfuric acid (Venus has clouds of sulfuric acid).

If people were to visit any other planet, they would need to bring their oxygen with them and have space suits which simulate the atmosphere on Earth, which is the one we evolved to breathe.