This dish can be made with only zucchini or only yellow squashes (instead of both) by using two pounds of either.
Coat a large skillet with oil. Add a third of the garlic, and heat over medium-high heat until sizzling, 1 to 2 minutes. Add a third of the zucchini and yellow squashes, and cook until just wilted and browned around edges, 2 to 3 minutes. Transfer to a plate. Repeat twice with remaining garlic, zucchini, and yellow squashes. Stir in vinegar to last batch, and let evaporate slightly. Return all batches of zucchini and yellow squashes to skillet, and stir in oregano. Serve warm or at room temperature. Garnish with oregano sprigs.