Preheat the oven to 400°F.
On a baking sheet, toss the zucchini, yellow squash, and red onion with 2 tablespoons of the oil, the oregano, and 1/2 teaspoon each salt and pepper. Roast, tossing once halfway through, until tender, 13 to 15 minutes.
Spread the baguette slices on a separate baking sheet, drizzle them with the remaining 2 tablespoons oil, and season with salt and pepper. Bake until lightly golden and crunchy, 6 to 8 minutes.
In a small saucepan, heat the balsamic vinegar over medium-high heat. Once it boils, reduce the heat to medium low and cook, stirring occasionally to keep from burning, until the liquid has become thick and syrupy, 8 to 10 minutes. Immediately remove the pan from the heat.
To assemble the crostini, spread each toasted baguette slice with the pesto and arrange them on a platter. Evenly sprinkle the roasted vegetables over the crostini and then top with the crumbled feta. Drizzle the entire platter with the balsamic reduction.