Congratulations! How high is the oak tree's canopy if one measures it from the ground up? Just a guesstimate... to put things in perspective.

Actually, growing climbing hydrangeas up trees is a long established method. It is a far superior growing surface to any sort of trellis - these are not exactly trellis-suited climbers :-) Any sort of wide, rough textured surface works well - a good sized tree trunk, wooden fence, masonary walls, cedar/wood siding, brick chimneys, etc. Something that provides enough purchase for the rootlets to adhere to.

If you check Google images for "hydrangea petolaris", you will see a number of photos of this climber growing up trees.

Here is a link that might be useful: images of climbing hydrangea

Martha Stewart talked about her climbing hydrangeas in a recent blog where she said she was forced to cut down the trees after Storm Sandy.