They are oak seedlings.

Seconded. If you pull some up you'll find he acorn on the bottom. There are lots of acorns visible in the picture.

So, I take it that oak seedling leaves don't look ANYTHING like mature oak leaves?

Depends on the type of oak, I think.

Do you have squirrels?

Oaks are notorious for hybridizing between species. You never know exactly what might pop up.

As I said, directly over these seedlings are live oaks. Live oak leaves are oval. These are very much not. But as I said, there is a red oak adjacent (not overhead), and those leaves are a little more like these. But there are squirrels, and acorns can certainly get moved around. Actually, in the lawn under the red oak, there are many fewer of these seedlings. This is the first year in decades I've seen these in such abundance.

Hybridization is an interesting possibility, but they do so within their own groups. That is, red oaks and live oaks don't hybridize with each other.

Intersectional hybrids are not unheard of.

Juvenile oak leaves are often different than mature oak leaves, especially in the red oak group. These appear to be red oaks of some kind. Certainly not live oak. Although live oak seedlings may have leaves with a few barbs on them.

That's a useful fact, that juvenile red oak leaves are different from the mature parent. That's probably what's going on here. My impression is that most of our live oak seedlings come up as sprouts from the roots, rather than from acorns. But it was a HUGE mast year, and we had piles of acorns all over the place.

Plenty of live oaks here, and I have a live oak in my yard too - seedlings never have lobed leaves like that.

Ugh - and are yours blooming? It's happening here right now and it's a scourge - pollen and catkins on EVERYTHING causing allergic reactions too. I foresee a bumper crop of acorns...