Preheat the oven to 400 degrees F (200 degrees C). Grease a baking sheet with olive oil.
Place salmon fillets on the baking sheet. Whisk mustard and honey together in a small bowl. Brush some of the mustard mixture all over the fillets, reserving some of the mixture in the bowl. Sprinkle marjoram, salt, and pepper over fillets.
Bake in the preheated oven until fish flakes easily with a fork, 10 to 15 minutes. Serve on warmed plates and top with the remaining mustard mixture.