How should I store oak cubes to be used later?

There could potentially be a transfer of water between the air and wood if not kept in bag (e.g. controlled environment), but I don't know if this matters. Moreover, white oak anatomy includes closed tyloses which are cells that resist water and rot and are why white oaks are used in cooperage.

I don't imagine there would be much loss of functionality (i.e. oaky-ness) over the time scale you would likely keep them. I would store them in the dark in a zip-top bag. If you are concerned about a loss of flavor, make a tea and taste it before you toss them in you brew after prolonged storage.