The pleural cavity and the airway are separate compartments; there is no anatomical path between them.
(Image source: https://upload.wikimedia.org/wikipedia/commons/0/0d/2313_The_Lung_Pleurea.jpg. Attribution: OpenStax College / CC BY (https://creativecommons.org/licenses/by/3.0))
Thus air in the pleural cavity would not go out through the mouth or nose. But it would make breathing difficult. How?
In a normal human being, the chest wall and diaphragm move outwards to bring about inspiration. Remember, though, that they are separated from the lung by pleural fluid. Since pleural fluid (like most liquids) does not compress or distend to any significant degree, the outward movement of the chest wall and diaphragm tends to create a negative pressure in the pleural space. This in turn expands the lungs. The reverse happens during expiration to increase pleural pressure and contract the lungs. In a sense, the pleural fluid 'couples' the movement of the chest wall to movement of the lung.
This coupling action would be lost with air, which is highly compressible and distensible. The bulk modulus of water (the main constituent of pleural fluid) is 2.2 GPa: about two million times the isothermal bulk modulus of air! With air in the pleural cavity, chest wall and diaphragm movement would do nothing to change lung volume. Rather, the air would happily accommodate to the new volume of the pleural cavity, without any pressure change.
As far as the diffusion of oxygen into pleural fluid is concerned, I believe it does happen (similar to what happens in extracellular fluid in any other location). However, I could not find any data on pleural fluid oxygen tensions specifically. For perspective, the interstitial fluid oxygen tension in different human tissues ranges from 3 to 7 kPa. I'd expect it to be higher in pleural fluid because of its close proximity to alveoli.