The heavy clay soil part doesn't sound correct for this species, but everything else does, like it growing right next to a Black Jack Oak, growing on a hill, both of which point to a dry well drained growing site. My best guess is you have Sand Post Oak (Quercus margarettae) based on the leaf lobing and general shape or this could be just an unusual leaf from a standard Post Oak (Q. stellata). If you can see a small twig(1st or 2nd year twig, so the outer most part of a branch) on this tree look to see if it's hairy or smooth. If it's hairy then you have Post Oak, if it's smooth then you have Sand Post Oak. The Ozarks(Southern MO, Northern AR, SouthEast KS, Eastern OK) and much of the Cross Timbers(North Central TX, Central OK, and Central KS) are dominated by almost pure stands of Post Oak with Black Jack Oak mixed in, in the driest most nutrient deficient soils. The soil in all of these areas where Black Jack Oak occurs are sandy or soils derived from sandstone. In other parts around here (Eastern KS), they grow in association with each other only on relatively dry sandy sites. When I say sandy, I'm talking about decayed sandstone which is made up of extremely tiny sand grains and sometimes the sandy part is only noticeable if you pinch some soil between your fingers and you feel the grit of the fine sand rather than the smoothness or slipperiness of clay(at least when it's moist). Also, when the soil is very dry it's still able to break apart without great effort, in contrast to more clayey soils, which can be like concrete when very dry and only breaks apart with great effort.

Campsis radicans has much more regular, pinnate leaves. Ampelopsis arborea occurs in Oklahoma. Here is a link that might be useful: Ampelopsis arborea

you need to learn how to insert finger and FIND OUT when your plants need water ... and/or when your soil needs water ... you have to learn how to NOT guess ... every 3 or 4 days.. may or may not be good enough ... and that is why relying on lawn sprinklers or some such.. often fails ... once you figure out how moisture moves thru your soil.. you might be able to use an automated system ... but then you will have the problem of some of the plants being too wet.. and others too dry ... so.. i suspect transplant shock.. related to less than stellar watering ...[you didnt really tell us when they were planted] but 2 thumbs up for noting the new growth is unaffected ... and .. to me.. that means you are close to getting it right .... if new growth was impacted.. you would be looking at serious problems ... btw .. once fully established ... once they have gotten over transplant shock.. you may have to adjust watering again ... ken

when were they planted ... perhaps they are perennial for you.. but for me.. they are warm annuals .... if you just bought them .... did you harden them off to sun before slamming them into the sun??? often.. we dont know where the plants came from ... perhaps a greenhouse.. so when you get them home.. you need to harden them off to full sun.. just to be careful ... did you apply any water sol fert after planting.. that can also burn tender young foliage ... i would probably just watch them for a few weeks ... and then look close.. and see if new growth is unaffected .. and if so ... then the problem may have just been transplant stresses ... plus that above ... ken ps: i had trouble with black or dark mulch.. but that was more of a high heat of summer problem... when it accumulated and retained heat .. late into the night.. basically causing a watering problem during the day ... and the slow night time recovery period ... i wouldnt expect it to be a spring problem ...