On the ISS, the breathable air has to include oxygen, but it can't be pure oxygen for safety reasons. So what is the composition of breathable air on the station?
You can monitor atmospheric composition, pressure, temperature, and so on of the several station's modules live via ETHOS (Environmental and Thermal Operating Systems) Life Support System Display, part of the ISS Live! pages. It won't always connect, so I'm including a relevant print screen for current data here:
ISS Live! Environmental and Thermal Operating Systems Life Support System Display. Click on image for live data.
As you can see, atmospheric composition is mean sea level Earth-like, meaning it is maintained at roughly 78% nitrogen, 21% oxygen, carbon dioxide produced by astronauts and live animal and plant biologic experiments is kept at small trace values (maximum of 6 mmHg) and scrubbed by the Vozdukh system in Russian Zvezda module, pressure is currently 749.56 mmHg (14.5 psi) with acceptable range between 14.2 to 14.9 psi, and temperature is, well, user friendly 65 to 80˚F (18.3 to 26.7°C).
Other gases present in trace amounts in Earth's atmosphere are not provided by the system, but are not excluded due to possible outgassing of equipment and experiment materials (where their composition and amount are considered safe for flight to the station during pressurized cargo selection and packaging), or as metabolic byproducts (ammonia, methane,...), and humidity is kept low at 10 ± 5 mmHg.
For more info on the Temperature and Humidity Control (THC) system, and some additional sources of information, see my answer to Air temperature and humidity inside the ISS.