In a large, straight-sided skillet, heat oil over medium. Add onion and garlic; cook, stirring occasionally, until onion is softened, 8 minutes. Add tomatoes and juice; cook until liquid is slightly reduced, 10 minutes. Reserve half the tomato sauce (about 3 cups) for another use.
Add potatoes, olives, and 1/2 cup water to skillet with remaining sauce. Partially cover and cook until potatoes are almost tender, 8 minutes. Stir in parsley. Season salmon with salt and pepper; nestle among potatoes and sauce. Cover and cook until fish is opaque throughout and potatoes are tender when pierced with a knife, 10 minutes. Serve salmon with potatoes and sauce.