Heat the oven to 425°F. Line a large rimmed sheet pan with parchment.
Toss the potatoes, tomatoes, and onion with 2 tbsp olive oil, oregano, garlic powder, 1/2 tsp salt, and pepper right on the pan. Spread in a single layer (crowding steams instead of roasts — use two pans if needed).
Roast 15 minutes, until the potatoes are starting to soften.
Push the vegetables to the edges. Pat the salmon dry (dry fish browns; wet fish steams), rub with the remaining 1 tbsp oil, and season with the remaining 1/4 tsp salt. Nestle among the vegetables, skin-side down.
Roast 10 to 12 minutes more, until the salmon flakes easily and the thickest part reads 145°F on an instant-read thermometer.
Scatter the olives over the pan, squeeze the lemon juice over everything, and shower with parsley. Serve with the lemon wedges.