Barrel aging question

I have a 5 gallon whiskey barrel that I got from a local craft distillery. I have a big RIS fermenting that I'm going to age for a week or two in the barrel to get add some whiskey character to it. I'd like to split the 5 gallon back and bottle about 2 gallons of it before I age it, and then age the other 3 gallons in the barrel... Would only partially filling the barrel be a bad idea? I'm thinking if I purge the interior of the barrel with C02 before I add the airlock it might help? I realize that a barrel isn't air tight, but it's only going to be in there for a week or two and hopefully it won't have too much oxidization.