Hello Ken,
If your oakleaf has just finished blooming, then the time to prune is right now. Since oakleafs bloom on old wood, this will give your shrub the maximum amount of growing time before next year's blooms emerge. You can cut your plant back quite hard if you need to- you could easily remove more than half its height without doing any permanent damage. As long as you leave it some foliage, your oakleaf should bounce back without much trouble. Be sure to cut back to just above a pair of leaves (or really, just above any node) using good, sharp pruners.

I think your best bet for transplanting will be to wait until temperatures are just beginning to cool back down in the fall (not sure exactly when this would be in your area), but well before any serious cold. When you move it, be sure to plant it high in the hole, and to give it a good watering once it's in. Good luck!

Here is one method for moving an established Oakleaf Hydrangea.

The safest time to transplant your Oakleaf hydrangea is in the late fall or late winter (provided the ground isn't frozen).
Avoid transplanting your Oakleaf in heavy soil, as these plants prefer well-drained soil. If your plant's new home has heavy soil, amended the soil well with organic material. Oakleaf hydrangeas are vulnerable to winter wind damage-choose a sheltered site for the best results.
The Oakleaf Hydrangea and its root ball will be very heavy-be sure to enlist the help of a friend or two when lifting and moving your plant. A sled, stone boat or wheelbarrow will make the job easier. I have used a tarp or heavy gauge plastic as a makeshift sled.
To avoid damaging the canes during digging and moving, you might consider bundling the canes loosely together and wrap them with burlap, a tarp, an old sheet, blanket or rug.
To dig up your Oakleaf, insert your shovel about 1-1½ ft away from the base of the plant. Cut through the soil with the shovel in a circular manner, lifting the blade to remove as much of the root ball as possible. Work the shovel under the root ball and use it to lift the plant. To avoid root damage from desiccation, move and replant your hydrangea as soon as possible. Dig a new hole as deep and slightly wider than the root ball. Once your hydrangea has been placed in its new home, add as much soil as required to fill in the gaps around the plant.
Water your plant deeply to settle the soil and hydrate the roots. As the plant is dormant, you probably won't need to water again during the winter, but checking it periodically for moisture would insure the rootball doesn't dry out.
In the spring, make sure the soil around the transplanted hydrangea stays moist (not damp or soggy) until the roots have a chance to establish themselves.
Adding a 3" to 4" layer of mulch over the transplanted Oakleaf's root zone will give it some extra protection throughout the winter. Leave a gap of 2-3 inches around the stems for air circulation.
I have used this method, successfully, for moving Hydrangeas, some so large that a winch line was necessary to remove them from their former location and ramp them onto a truck.
I haven't found it necessary to prune relocated dormant Hydrangeas, but I always try to dig them without severing many roots.

Good luck!

Thank you for the responses. Pruning the Oakleaf this weekend and will transplant when it gets cooler in September.

Just thought I would post an update for the help responses I received in case anyone was following. It is now Summer 2011 and I am happy to say that the transplant was a success! The Oak Leaf has responded well to the move and is just about to portray its gratitude with magnificent blooms. Thanx to all that helped last year.

Oh wow, that's great! That was very nice of you to come back to update everyone. =)

My oak leaf hydrangea is tall and and too wide for where it is. Can I cut it down or move it without killing it.

You can but it will have less transplant shock if you do this while the plant is dormant in winter. If transplanted, you may need to check it often in the first summer as it may complain then about the heat and water issues in ways it never recently did. Plan to move as much as possible of the root system as you can.

You could do either........but now is not the best time :-)

If you were to cut it back now, you would remove any possibility of flowers this season and perhaps even next year. And it would simply grow back to the same size (or larger) very quickly. Unless a dwarf form, these are big shrubs!!

Moving it is a possibility, but again the timing is wrong. Transplanting a large, established shrub just when it is expending a great deal of energy to push new growth is going to generate some serious transplant shock. That may not be insurmountable but it will certainly set the plant back. And watering this summer will be a major concern. Also, it is not recommended to prune or trim back the shrub before moving, as it needs all that foliage to regenerate the root system. This makes it far more difficult to move a large shrub during the growing season. If you can wait until fall when the shrub enters dormancy, you can trim it back proportionately (active growth has stopped; leaves falling naturally) and shrub roots in fall are already in a period of regular elongation, so far less chance of transplant shock and much better chance of reestablishment through the rest of the fall and into winter.

its always great when someone posts a follow up so we can see how things worked out.

And it's obvious they did just that for you.

Your oak leaf is gorgeous.

Wow! What a beauty! What variety is it?

Looks very happy where it is now!

Unfortunately, I do not know the exact variety. This Oakleaf was original to the house when the house was purchased. It had to get moved when we installed a deck in the back yard.

Here's the oak leaf 12 years after the transplant. It's crazy how big it's getting. Still going strong.

So pretty! do the flowers turn hint of pink later in the season? I imagine it will look magnificent in the fall too

I think all Oakleaf flowers turn pink. Maybe there are some that don't that I'm not aware of.

Yes the flowers turn pink