You don't need to worry about O2 during the active fermentation, in fact the incorporation of oxygen during the active ferment is needed. You see a lot of new wine makers franticly worried about keeping their musts away from oxygen and then wondering why they get stuck fermentations.
I ferment in 44 gallon trash barrels. I can fit 7 or 8 lugs in each with enough room for the cap. When the must is put in the barrel it comes up about 3/5 of the way. After a few days of fermentation the grape skins get pushed up by the CO2 produced by the fermentation, forming a cap that floats on the surface of the wine. You need to punch this down several times a day. Punching down serves several purposes. It keeps the cap wet so it doesn't spoil, it keeps the skins in contact witht he must so the colors and flaovrs that are in the skins come out <for the most part red wine isn't red whenyou crush it, it's clear, it doesn't get the red color until it's been fermented on the skins> , and vigorous punching down incorporates O2 into the must. I punch down 5-6 times a day (depending on what style I'm looking to make.) When the wine is dry you can either press then or leave it alone and let the cap fall and let everything sit for up to 2 weeks for an extended maceration. If you let the cap fall you need to be a little more vigilant about keeping a CO2 layer on top of the wine, but you still don't need to make yourself crazy. There is still LOADs of offgassing CO2 in the wine and you're fine for a few weeks.
If it works in 44 gallon barrels or 1 ton microbins it will certainly work in a 6 gallon carboy.
After you press, put the fresh wine in a container for a few days to let the gross lees fall out and then rack to your secondary. At that point you need to start being vigilant about keeping oxygen away. It's not your friend anymore. Up until that point it's not that big a deal.
Think of it this way. Why would you get all crazy about keeping oxygen away form your must when you are going to put it all in your press, squeeze the bejesus out of it and then pour it into a container? Don't you think it gets pretty well oxygenated when that is going on?
When you put it into your secondary you want it right up into the neck. I generally have enough around so I dont need to worry about using marbles to take up the space and I can just juggle wat I need between different containers. ( I crushed just under a ton this fall so I end up with a LOT of different vessels. Having a bunch of smaller sizes is key.) If you don't want to be bothered with the whole marble escapade just pour in what you need from a similar commercial product. A glass or two of Yellow Tail added into your carboy to raise the level won't ruin your batch. Just don't tell anyone.