Answer: Oakleaf hydrangea has few leaf disease problems. It would be helpful to have more information on the size, location and shape of the spots and on which leaves they are found (old vs. young).
Fungal and bacterial leaf spots are usually seen first on older leaves as these have been around for the longest time, allowing for development of the organism causing the spot. Your indication that spots were all over the plants seems to indicate that young leaves were also affected.
If so, the culprit may be something other than disease. Chemical burn from a spray application or something unusual like products used to wash brick on the siding of a home or your roof can cause such spots.
Make sure the plants are not lacking for water as new transplants take time to develop a root system that can support the top (especially in this HOT weather!). Let us know if there is additional information about the condition which might help narrow the possible cause--feel free to submit another question via the web site.
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