PREHEAT the oven to 425 degrees. Bring a large pot of water to a BOIL.
COOK pasta in a large pot of boiling salted water until tender to the bite, about 10 minutes.
DRAIN and RINSE under cold water until cool.
For the Dressing: WHISK together vinegar, oregano, mustard, lemon zest and juice, garlic, salt and pepper in a large bowl. ADD olive oil in a slow, steady stream, whisking until smooth.
For the Zucchini and Yellow Squash
CUT zucchini and squash into 1-inch chunks.
PLACE on a parchment-lined baking sheet. DRIZZLE with olive oil and TOSS to coat.
SPREAD evenly on the baking sheet and SPRINKLE with salt and black pepper.
BAKE zucchini and squash for 20 minutes.
TURN the pieces over and BAKE for an additional 10 minutes, or until the zucchini and squash start turning brown.
REMOVE from the oven and let COOL.
For the Salad
ADD pasta to a large bowl. ADD dressing and TOSS to coat.
ADD the zucchini and squash, onion, parsley (if using), nuts, olives, and feta cheese. STIR to combine.