Fortunately for homebrewers there are convenient ways to move beer around without ruining your homebrewed suds with the ill effects associated with oxygen.
As you mention in your question, one handy method to help reduce oxygen pick-up during racking is by using carbon dioxide as a blanketing gas. While this method is handy, it does require you to actually have bottled carbon dioxide laying around for use. (I will assume that suggesting other blanketing gases like argon and nitrogen are not of interest to you, so I won’t discuss them here.)
The best way in general terms to limit oxygen pick-up during racking and bottling is to fill the beer from the bottom of the container and then to limit the amount of headspace in the container by matching your container size to the amount of beer you have on hand. Using a solid racking tube to deliver beer to the bottom of the container being filled is a simple and reliable method to control turbulence during filling. Once the beer has been racked it is helpful if some carbon dioxide gas is produced by yeast because this will help scrub the headspace of oxygen. Racking with some residual extract is the best way to help this process happen.
Another important consideration is the oxygen barrier properties of the secondary fermenter. While it is acceptable to ferment beer in plastic containers, I would avoid aging beer in a plastic secondary because ordinary plastics allow oxygen to travel across the container wall and into your beer. Not the ideal situation.
The challenge of oxygen pickup pops up again when it is time to move your beer from the secondary to the final container. If the container is a keg you can fill the keg from the bottom using the tube in the keg for filling. But most homebrewers who keg have carbon dioxide containers, and I am guessing that you don’t have this set up. This means that you are most likely bottle conditioning your homebrew and need to rack your beer from the secondary to a bottling bucket, and then into your bottles. This is the step in the brewing process where real damage from oxygen often occurs.
The first challenge is to move the beer from the secondary to the bottling bucket. Unlike the transfer from the primary where some fermentation is happening, the beer at the end of secondary is done fermenting. My advice is to keep the time investment to a minimum. Start by preparing your priming solution and pouring into the bottling bucket, then, fill your bottling bucket with beer using your racking tube and quickly bottle. At home this is the method to use when you do not have pressurized containers.
Commercial brewers do things a bit differently. Even brewers who bottle condition fill their bottles with some level of carbonation in the beer. This allows the beer to be foamed or “fobbed” before the bottle is capped. Fobbing pushes air from the headspace and is a very effective method used to reduce the oxygen content of bottled beer. In order to do this the beer must be stored in a pressurized vessel, such as a keg during storage so that some level of carbon dioxide remains in the beer.
You ask a question with a short and simple answer. The fact is that without using carbon dioxide as a blanket gas and pressurized storage containers for secondary fermentation and/or bottling containers it is difficult to really control oxidation.