The dark pellets are frass (insect poop), and given their size, shape, and abundance, they were probably produced by caterpillars. The other objects pictured are molts (shed skins) from caterpillars, in this case probably Orange-striped Oakworm Moth caterpillars based on the host plant and presence of the horn-like projections on their head. Oakworms are a native insect and they feed in groups on oak foliage. Even if they defoliate a young tree, they do not cause the plant enough damage to worry about, and it will leaf-out normally in spring as long as an unrelated issue doesn't damage the roots or trunk. For mature trees, any caterpillar feeding isn't consequential (trees can afford to lose foliage every year to insects, infections, and storm damage), even if the caterpillars are abundant. No pesticide use would be practical or recommended for this situation, and it would risk harming other insects in the process.

Fortunately, plenty of predators will eat caterpillars and reduce their numbers over time, like birds, paper wasps and hornets, and stink bugs; bats can eat night-flying adult moths, and some caterpillars fall victim to insect-specific infections like certain viruses and fungi.

Since you've been seeing evidence of their presence for a couple of weeks now, it won't be too much longer until the caterpillars mature enough to stop feeding and wander off to pupate. You can sweep or hose off the frass and shed skins as needed, but they should stop appearing in another week or so.

Miri