If I'm not mistaken, I see helianthus and sedum happily blooming there, which means you do have pretty good sun there.
Personaly I wouldn't be looking to plant any hydrangea in such setting, especially in OK, with your heat.
You already have watering problem because of the tree, but heat reflected off the wall will make 'hydrangea proposition' even more challenging.
The only hydrangea, I guess, will be able to sustain proposed conditions is one of the H. paniculata family, but unfortunately most if not all of them will be much larger than alloted space. Of course, you could annualy prune them to keep in check, but still by the end of the season they'll eat everything else there.
Why wouldn't you use tall phlox, David for example, which will be very happy there and bloom nonstop (with deadheading) from July till frost?

I appreciate you taking the time to advise me. Although not apparent from the photo, the "gap" is about 3 feet or so back from the Helianthus (Angustifolia btw), so not as much sun as it first appears. Because of a soil born water fungus here (Phytopathora), Phlox do not survive. I've seen Oakleaf Hydrangeas around town so I know they can take our heat/humidity. A good point about the reflected sunlight off the wall. Thanks for your input.

i also see a h. paniculata 'pee wee' listed. is this right?
would there be the same cultivar name for two different species of hydrangea?
if so, maybe that one would fit the size of the site and could handle the afternoon sun.
i have a 'limelight' paniculata that does not seem to want to grow as large as george's do in one season and i prune it selectively each spring. ;-)
you could actually shape one into tree form and let it hang forward over the perennials there.
are you set on a hydrangea for this spot, maryl?
i grow glossy abelia in a spot which has similar
conditions and it does well in our heat and afternoon sun, blooming from mid-summer on. thought not as showy as hydrangea flowers, they always attract butterflies and the shrub has a more airy feel to it when the branches are allowed to naturally arch.

I don't know about Paniculata having a cultivar named 'Pee Wee'. The Hydrangea I'm refering to is Quercifolia 'Pee Wee' as described in the Wayside catalog. This variety shows it can take more sun then other Oakleaf types. Obviously I have no personal experience to verify the Wayside cultural blurb about Pee Wee, but was hoping someone else did....As for Abelia, yes I've thought about it. I'm not overly fond of it, but after 20 years of experiments my options are running out, so thanks for reminding me of it again. I really meant it when I said all suggestions are welcome.

Here is a link that might be useful: Wayside Hydrangeas

If you'll be considering abelia, take a look at cultivar "Edward Goucher'. Pale lavender flowers and slightly smaller than a. grandiflora.

this site below says 4'-6' tall. might be a better choice than oakleaf for afternoon sun.
as for oakleafs and sun, a local wholesale nursery has a raised bed of them at the entrance to their business that are in all-day sun and they are enormous and flower profusely.
the flowers change to their darker colors much more quickly in that setting, as do the leaves, but they are extremely showy plants. of course, i assume they are well irrigated by the gardeners who maintain the beds and that allows them to thrive in such a hot location.
could you tuck a viburnum in there?
i have a burkwood viburnum and like it for the highly fragrant spring flowers. it is an open shrub(unpruned), growing taller than wide and has been rather slow growing so might not outgrow your space for a while.(?) i understand they do get large over time but can also be pruned.

Here is a link that might be useful: h. paniculata 'pee wee'

Jeff, interesting, I've never heard of PeeWee paniculata either. I took a look at the picture in the provided link and noticed some discrepancy, in particular they claiming it to be a smaller PG, but picture show flowers composed of both sterile and fertile florets, while PG has only ferile flowers. To me it looks more like a Kuyshu than a PG.

You all are so helpful. Thanks for the link on Paniculata Pee Wee. If Paniculata takes full sun then the few hours of western sun it will get may not be enough. But I will do some more research. What is a Kuyshu btw?..........You know I have both of Dirrs books on trees and shrubs, and still have only come up with the Oakleaf as perhaps being suitable for this gap. So many shrubs discribed take acidic soil, and we just don't have that here.......I like Viburnums (grow the ubiquitous "Snowball bush"), but no, they would grow way too large for this small area. Heck, I've even thought of trying Zebra grass, but again don't know about the part shade issue.

I did some more research on Paniculata Pee Wee and it may be a better choice. Saw a nice photo of one somewhere.... Is it correct that all the Paniculata's blooms are produced on new wood?....Thank you again....Maryl

Yes, that's correct.
They bloom on a new wood which means they are 'prunable' in early spring.

That's better then old wood. Too many unexpected hard freezes here to count on blooms from the Macrophylia. Incidently, I see that Forest Farm (mail order from Oregon) has not only the Pee Wee Paniculata Hydrangea, but different varieties of it as well. Do the Paniculatas leaves change color in fall?

I ordered Pee Wee Paniculata yesterday. Will see how that does. Thank you for all your help and suggestions.