Preheat oven to 400°F. Spray a rimmed, light-colored 18 by 13-inch baking sheet with non-stick cooking spray.
Put the 2 1/2 lbs Yukon Gold potatoes (cut into 1-inch pieces) in the center of the prepared baking sheet. Drizzle with 1/3 cup olive oil, sprinkle with 1 1/2 tsp dried oregano and salt and freshly ground black pepper to taste, then toss until the potatoes are evenly coated.
Spread the potatoes into a single, even layer across the baking sheet with a little space between pieces.
Roast in the preheated oven, tossing once halfway through, until the potatoes are nearly tender and just beginning to brown lightly, about 30–35 minutes total. Check doneness with a fork.
Remove the baking sheet from the oven. Move the potatoes toward the center, drizzle with 1/4 cup fresh lemon juice and sprinkle with 1 Tbsp minced garlic, then toss gently to coat.
Spread the potatoes back into an even single layer and return to the oven. Roast until fully tender and golden brown in spots, about 10 more minutes.
Remove from the oven, sprinkle with 3 Tbsp minced fresh parsley, toss once to distribute, and serve warm.