It is impossible to eliminate all O2 exposure. The best practices work to eliminate as much as practical or feasible.
The fact that the surface of the wort (or, more properly the beer) is in contact with air isn't nearly as bad as splashing the beer. The agitation really helps push the air into the beer, causing the oxygen to go up. As well, surface contact with air OVER TIME will allow the oxygen to be absorbed into the beer.
Some of the common methods of preventing oxidation are to reduce the amount of time and surface area by using a carboy vessel for secondary. For primary, it's not as important because the fermentation pushes the Oxygen out, leaving CO2 in the headspace. Racking to a secondary *might* cause some Co2 to come out of solution, thereby pushing some air out of the headspace, but there is no guarantee. It's good to purge the headspace with CO2 if you have a kegging system on hand.
The racking cane might have a poor seal, allowing some air to enter the tubing, causing some oxidation. Check to see if you are getting bubbles in the tubing where there are any joints. CO2 coming out of solution during racking is normal and needs to be differentiated from air entering the system.
Bottling can be a place that causes oxidation. Filling bottles too quickly, causign splashing might introduce some air, and having some headspace might also be a place O2 enters the beer. Make sure to fill gently and cap on the foam if you can, otherwise O2 Barrier Caps are a good idea.
Oxidation is not generally a problem for any careful homebrewer. There is a time period before it really show up in the beer and temperature plays a large part as well. A careful homebrewer who bottles and keeps the beer cool after carbonation is not likely to make beer that has noticeable oxidation. A homebrewer who is rough when handling their beer, and isn't careful about purging headspace and who keeps their bottles warm until serving, might start having oxidation flavors develop a lot sooner.
Note that I am talking about beer, not wort. Wort is the beer before fermentation, during which the O2 is used by the yeast. Having oxygen in the wort is a good thing. Having it in the beer is not.
Also, keggers generally have less problem because they tend to purge the headspace in the keg, even if they don't chill the keg right away. But, in general, they also tend to put their kegs in the kegerator sooner.
