Funny you should say this i'm giving a bunch to a buddy of mine to see if they can take off at his property. I'm just worried about damaging the roots that they won't grow for him ya know. Thats why I thought about buying good stock.
Even so, I'd be looking for the smallest sprouts you can find. They grow down pretty quick looking for water.
At my place, you can find the oaks when they're about 6" tall and have a cluster of leaves on top (the ones the deer don't nip off). If you dig down about a foot the tap is starting to get fine enough to where I'd imagine breaking it off there wouldn't be a big stress on the tree if it was watered in well at transplant. I'm not sure a potted seed grown oak would have any better odds (the pots aren't deep enough). I've found those tend to have j-rooted taps and that's probably harder to overcome than acute root damage.
I'v got really nice Bur Oak seedlings in half gallon milk cartons with tap root growing out the bottom. Can usually dig them with 15-20 inches of tap root.
They are like a large plug; you get them quite wet (to hold root and soil together), slit down the side - and around the hole where the tap root extends out the bottom of the carton with a box cutter and plop them into the ground. Unfortunately, the shipping charges would probably be outrageous. They get a nice start since the tree seedling and root ball are proportionate; they take off well the second year