When you dump the trub, some O2 will indeed be pulled into the fermenter. But since O2 is lighter than CO2, your beer will still be separated from the O2 and well protected.
Typically, when you do anything to disrupt the beer, such as pulling trub, you will cause some of the dissolved CO2 to escape. This in turn will push any O2 that has entered the fermenter to be expelled.
The other thing to consider is related to getting O2 into your beer. While the beer is still actively fermenting during the primary and secondary stages it is metabolizing in an anaerobic state (no air) because when air is present the yeast will quickly absorb it to build a cell wall and ultimately bud new cells. The point is, if your yeast is active (which it should be) any minor amount of O2, which might enter the beer, would be quickly absorbed by the yeast.
Where O2 really becomes a problem in brewing is during transfers to secondary fermenters, kegs, or bottles, which are "full of air". In these instances, there may be too much O2 for the yeast to deal with (because of the lack of sugar at this stage). If the yeast is overwhelmed, then you can get oxidation of the beer - and find that wet cardboard taste in you finished beer.
The bottom line: be cautious and try to minimize O2 exposure. If you are filling a large vessel, purge it with CO2 first. But don't sweat things like dumping trub or dry hopping. These are relatively low risk events relative to oxygen contamination in your beer.