We actually already know of one, but it's a moon, not a planet!
And yet it is so thanks to a small loophole.
This is Europa. It's a moon of Jupiter and is slightly smaller than our Moon. Its surface is composed of layers of ice, probably with liquid water deeper down. On the ice, this moon is constantly bombarded by deadly radiation (caused by the nearby volcanic moon Io and Jupiter's immense magnetic field).
Its atmosphere is composed almost exclusively of oxygen. But oxygen is so rare in the universe, so where does it come from? While oxygen on Earth is formed by plants, the oxygen on Europa is formed by radiation passing through the ice, splitting water molecules into hydrogen and oxygen gas (the hydrogen escapes into space).
But you couldn't breathe in it! Why?
Because Europa's atmosphere is actually very, very, very thin. The air pressure at its surface is only a billionth of that on Earth. So it can barely be said to have an atmosphere.
So here's the loophole: You asked about a planet whose atmosphere is 99% oxygen, which is a ratio. But you didn't ask how much atmosphere there is in the first place.