One always leaves some small amount of air space in the barrel. The wine will expand and contract for many reasons. Temperature and atmospheric pressure changes, for example, can pull the airlock fluid into the barrel, or push wine out of a fully filled barrel. The barrels in the photo, while very colorful and photogenic, are not the proper** way to do it.
To avoid oxygen contact, you can top up the barrel with a neutral gas. You can buy bottles of many different gas options. Some prefer noble gases, such as Argon, but I prefer Nitrogen myself.
Hope this helps.
** Edit: Perhaps a better term I could have used would be "trouble free". Because full topping up may be traditional, but not trouble free. That is, if you are there, every day, monitoring the air locks, so when things like the photo happen, and you immediately clean them out and replace with fresh air lock fluid, then it can be done. Leaving a little air space, and using modern methods, you can leave for a month or more, especially in winter, and things should be okay.