Clearly, the planet has lots of life on it, which includes life that produces oxygen as a waste product. On Earth, that's mostly done by plants, but a different planet might be differently arranged.
You need life to be doing this, and for there to be a lot of it, because free oxygen is too reactive to stay in an atmosphere over geological time unless it is refreshed. On Earth, the initial atmosphere had no free oxygen, but early life began to produce it. After it ran out of things it could easily react with, it began to accumulate and The Great Oxidation Event, which ended about two billion years ago, was when the atmosphere started to contain free oxygen.
The problem with your atmosphere is the 10% of carbon dioxide. That's a lot, and it would produce a huge greenhouse effect. That would likely run away, producing an atmosphere rather like that of Venus. Your translation of 2000ppm in Earth's Carboniferous atmosphere to 10% is wildly wrong: it should be 0.2%. That will produce a warm climate, but it will support life.
With that mistake corrected, your atmosphere seems perfectly plausible, for a planet at the correct distance from its star.