| Yes, it is safe. There was no plant contact with the tree that high up. The pollen of the flowers do not float in the air. Even if it did, there is no allergenic oil in the pollen anyway.
The allergenic oil (it's in the plants resin) is not volatile. (it does not evaporate, condense and then land on things).
To sum it up, direct contact (with the tree trunk, limbs or leaves of the tree you cut down) with the resin of the poison oak plant from a wound in the trunk, stems or leaves of the poison oak plant would have to occur for the hickory tree to be contaminated.
My credentials: I wrote the "Poison Oak & Poison Ivy Survival Guide." Sandra