Kate,

Oak trees are resilient and have large tap roots that may descend as high as the tree rises, digging a little round the base should not do any real harm.

I would transplant a bit earlier, give the plants some tme to re-establish before the onset of winter. Prepare your soil with a good deal of humus, removing weeds and competing roots from the tree( a few small bits removed should not harm). Water your plants well the day before lifting them. Transplant quickly with quite a bit of soil. Do not let the roots dry out, they should be fine. Keep well watered for a couple of weeks

Cheers Greenmanplants

hi kate,
i agree with all greenmanplants said. i have just transplanted about 100 plants or so and must do twice that many (at least) more in the next week. these are amazingly resilient plants, and certainly the oak will be just fine. in mid tn we haven't had much rain onsite and the process is going quite well. plant in evening or early morning as you have time. most hellebores won't even show signs that they've been moved. the roots are significant, so try to dig as much below ground as you see above ground. in most cases it won't be as drastic but better to be safe if possible. i water the plants the day before transplanting and immediately after... then as needed for the next week or two. i grow mostly in sun adjacent to oaks so no worries. and i second the advice to move them earlier if possible just so they'll establish before winter.

I have just moved 20 plants in Melbourne (Aussie). (It's winter here with daily temps 12-14 C and nights down to 5-7 C) I cut off most of the older leaves so that only flower buds and new leaves were left, dug out a wedge of soil and potted them into 8 inch pots. So far so good!!!