As a whole air purifiers do not increase oxygen. They do not contain any oxygen generating equiment. There is one circumstance when an air purifier will increase oxygen concentration in the home. This is when a duct through an exterior wall is connected to the input of an air purifier. The air purifier will then take the very slightly more oxygenated air from the outside purify it and pump it into your home. Most people do not do this but an example of such an attachment can be seen in this article.