Any time you open or remove liquid from the fermenter you are exposing the beer to oxygen unless you take measures. However, often the effects of this are low.
When fermenting, your beer will naturally produce carbon dioxide (CO2). Since CO2 has a higher density than air, a natural layer of CO2 will form above the surface of your beer, minimising exposure to air introduced to the fermenter when sampling or dry hopping.
When you disrupt the beer in this manner, some dissolved CO2 is likely to escape, forcing the oxygen out of the fermenter and a small amount of oxygen can be absorbed by the yeast if dry hopping during active fermentation. As with any fermenter, you should be careful to minimize oxygen pick-up as much as possible once fermentation has begun.
With dumping the yeast, has a higher chance of introducing oxygen due to the volume removed from the fermenter is usually more and faster than CO2 production during active fermentation. therefore, it is recommended that positive pressure with CO2 is applied to the top of the fermenter when dumping yeast and dry hops.