What is the partial pressure of oxygen in a balloon?

The partial pressure of oxygen will vary according to where you look. Atmospheric oxygen partial pressure is approximately 21% of the atmospheric pressure of the location at which you measure (typically around 1 atmosphere at sea level, making the partial pressure of oxygen at seal level 0.21 ATM or 21.3KPa). The value varies geographically and with time, but also varies in different tissues of organisms, since not all oxygen available is absorbed, and complex multicellular organisms will have certain tissues (respiring muscle) that use up oxygen, resulting in a lower partial pressure there.

normal oxygen partial185.4 kPa

or in another unit:

100 mmHg in the arterial blood. The partial pressure on the alveolar site is about 105 mmHg.